Let us go back now to a most curious fact about the Latin word lingua, from which many of the words described in this section are derived, namely that the word lingua is a historical cognate of the English word tongue, since the two have the same Indo-European source. It does not seem likely, but the common ‑ngu‑ letters give it away. The two words look even less similar if we compare the sounds, only one of which is shared, namely the velar nasal [ŋ] (cf. Part I, Chapter 7).
These two words present an excellent example of what historical linguistics and language reconstruction has achieved since the 18th century, when William Jones first suggested the genetic connection between Germanic and Latin languages (cf. Part I, Chapter 3).[i]
By using the tools of comparative reconstruction (see Part I, §3.8), linguists have ascertained what the ancestor of these two words must have looked like in Proto-Indo-European. We know that English tongue [tʰʌŋ] comes from Old English tunge, pronounced [ˈtʊn.ɡe], with [ʊ] as in foot and [ɡ] as in get, and a final vowel, which was probably a schwa [ə]. In modern German, a language related to English, the cognate of Eng. tongue is Zunge [ˈtsʊ.ŋə]. Linguists have reconstructed the source of these two words in Proto-Germanic as *tungǭ. And by comparing that root with others in related languages, such as Irish teanga, Latin lingua, Polish język, and Sanskrit जिह्वा (jihvā́), they have concluded that they all came from a root that looked something like *dnghu- in Proto-Indo-European (or, in a more detailed reconstruction, *dn̥ǵʰwéh₂s).[ii] We say that we have identified the root and not the word, with all its endings, since inflectional endings were many for each word and they changed more often and less predictably than roots did.
One of the most striking differences that we find in the roots of the daughter languages above is the beginning t in English tongue, which corresponds to the Latin l in Lat. lingua. This is striking because this is not a regular correspondence. We do not find other cognates in which English (or Germanic) has t where Latin has l. The reconstruction of a d in that position in Proto-Indo-European is consistent with the Germanic t, since PIE d became t in Germanic and, typically, stayed d in Latin. Thus English ten, with a t, is a historical cognate of Latin decem (Spanish diez), with a d. The same thing is true for countless other words, for this is a regular correspondence, which goes by the name of Grimm’s Law (cf. Part I, §3.8). The l in the Latin word is a real oddity. It should have been a d, for PIE d stayed d in Latin. In other words, the Latin word should have been *dingua, not lingua. So, what is going on here?
It turns out that we have evidence that Old Latin, as well as related Italic languages of the first millennium BCE, had a d in this word. In other words, the Old Latin word for tongue was indeed dingua, not lingua. So why did Classical Latin have lingua? This d to l change is not a general, regular one, but rather a sporadic one, but one that is not unheard of either. The sounds d and l are similar enough phonetically for the change to be a ‘natural’ one: both are voiced alveolar consonants, that is, they are both made with the tongue in the same position (same point-of-articulation), with the tip of the tongue touching the ridge behind the top teeth, and they are both consonants made with the vocal chords vibrating (voiced sounds) (cf. Part I, Chapter 7). The main difference is that for the l the sides of the tongue are not touching the sides of the mouth, which allows for air to escape through the sides (we say that l is a lateral consonant).
Sporadic changes are changes that appear occasionally for no apparent reason, as opposed to regular changes that apply to a certain sound in a certain phonetic environment. It seems very likely that the sound change did not occur in Latin itself, where it would have been one of a kind, but in a related Italic language, namely the language of the Sabines, an Italic people who had a shared history with the Romans. Presumably Latin then borrowed the lingua pronunciation for this word (borrowing is unpredictable, unlike sound change).[iii] That is why this non-etymological l, which actually exists in a few Latin words, is known as Sabine L. The contrast Sp. olor ~ Eng. odor is another such case of a Latin l where there should be an etymological d (cf. Part II, §37.4). It has been suggested that the borrowing of lingua to replace dingua might have been motivated by the fact that the word for lick in Latin, namely lingĕre, also started with the sound l and had other similar sounds to dingua (namely ‑ing‑). Lat. lingĕre is cognate with English lick, from Proto-Germanic *likkōną, from the Proto-Indo-European root *leyǵʰ-.
The New Latin word cunnilingus [ˌkʌn.ɪ.ˈlɪŋ.ɡəs], the same in English and Spanish (though cunnilinguo is also attested in Spanish), refers to the ‘stimulation of the female genitals using the tongue or lips’ (COED). This word is a late 19th century creation (between 1985-90) from Latin parts, though it is not a well-formed one and who the creator was is not known. It obviously contains the root ling‑, but it is not clear if it comes from Lat. lingua ‘tongue’ or from Lat. lingĕre ‘to lick’ (principal parts: lingō, lingĕre, linxī, linctum). Most likely it was a play on both of these words. The first part of this word comes from Lat. cŭnnus that meant ‘cunt’ (vulgar word for ‘vulva’), but also ‘woman’ and ‘female pubic hair’, from where comes the patrimonial vulgar word for the female genitalia in Spanish, coño ‘cunt’. As for the source of Lat. cŭnnus, there are different theories, none of which has been proven.
 There are many slang terms for cunnilingus in English, such as carpet munching, muff-diving, giving lip, lip service, and dining at the Y. Curiously, someone who performs cunnilingus has been referred to as a cunnilinguist. Other rare derived words are the adjective cunnilingual and the noun cunnilinguism the practice or habit of cunnilingus’ (WNTIU).
The analogous term for ‘stimulation of the male genitals using the tongue or lips’ is known in English as fellatio [fə.ˈleɪ̯.ʃɪ.o̯ʊ]. This is a New Latin term created in 1894 by Havelock Ellis, from the stem fellat‑ of the passive participle fellatus of the Latin verb fellēre ‘to suck’ and ‘to fellate’. The Spanish equivalent is felación, an adaptation of the same New Latin word. The main slang term for fellation in English is blowjob and in Spanish, slang terms include mamada from mamar ‘to suckle’ and chupada from chupar ‘to suck’.