Etymology
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fellatio (n.)
1894 (Havelock Ellis), from Latin fellatio, noun of action from fellatus, past participle of fellare "to suck," from suffixed form of PIE root *dhe(i)- "to suck." The sexual partner performing fellatio is a fellator; if female, a fellatrice or fellatrix. L.C. Smithers' 1884 translation from German of Forberg's "Manual of Classical Erotology" has fellator, fellatrix, and fellation, but not fellatio.
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fellate (v.)
1968, verbal derivative of fellatio. Related: Fellated; fellating.
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fellation (n.)

1887, noun of action formed classically from the past-participle stem of Latin fellare "to suck" (see fellatio + -ion).

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*dhe(i)- 

*dhē(i)-, Proto-Indo-European root meaning "to suck."

It forms all or part of: affiliate; affiliation; effeminate; effete; epithelium; fawn (n.) "young deer;" fecund; fellatio; Felicia; felicitate; felicity; Felix; female; feminine; femme; fennel; fenugreek; fetal; feticide; fetus; filial; filiation; filicide; filioque; fitz; infelicity.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit dhayati "sucks," dhayah "nourishing;" Greek thēlē "mother's breast, nipple," thēlys "female, fruitful;" Latin felare "to suck," femina "woman" ("she who suckles"), felix "happy, auspicious, fruitful," fetus "offspring, pregnancy;" fecundus "fruitful, fertile, productive; rich, abundant;" Old Church Slavonic dojiti "to suckle," dojilica "nurse," deti "child;" Lithuanian dėlė "leech;" Old Prussian dadan "milk;" Gothic daddjan "to suckle;" Old Swedish dia "suckle;" Old High German tila "female breast;" Old Irish denaim "I suck," dinu "lamb."

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blow-job (n.)
also blowjob, "act of fellatio," 1961, from blow + job (n.). Exactly which blow is meant is the subject of some debate; the word might have begun as a euphemism for suck (thus from blow (v.1)), or it might refer to the explosive climax of an orgasm (thus blow (v.2)). The oldest verbal form appears to be blow (someone) off (1933), a phrase originally among prostitutes.

Unlike much sex slang, its date of origin probably is pretty close to the date it first is attested in print: as recently as the early 1950s, military pilots could innocently talk of their jet planes as blow jobs according to the "Thesaurus of American Slang."
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head (n.)

Old English heafod "top of the body," also "upper end of a slope," also "chief person, leader, ruler; capital city," from Proto-Germanic *haubid (source also of Old Saxon hobid, Old Norse hofuð, Old Frisian haved, Middle Dutch hovet, Dutch hoofd, Old High German houbit, German Haupt, Gothic haubiþ "head"), from PIE root *kaput- "head."

Modern spelling is early 15c., representing what was then a long vowel (as in heat) and remained after pronunciation shifted. Of rounded tops of plants from late 14c. Meaning "origin of a river" is mid-14c. Meaning "obverse of a coin" (the side with the portrait) is from 1680s; meaning "foam on a mug of beer" is first attested 1540s; meaning "toilet" is from 1748, based on location of crew toilet in the bow (or head) of a ship.

Synechdochic use for "person" (as in head count) is first attested late 13c.; of cattle, etc., in this sense from 1510s. As a height measure of persons, from c. 1300. Meaning "drug addict" (usually in a compound with the preferred drug as the first element) is from 1911.

To be over (one's) head "beyond one's comprehension" is by 1620s. To give head "perform fellatio" is from 1950s. Phrase heads will roll "people will be punished" (1930) translates Adolf Hitler. Head case "eccentric or insane person" is from 1966. Head game "mental manipulation" attested by 1972.

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lesbian (adj.)

1590s, "pertaining to the island of Lesbos," from Latin Lesbius, from Greek lesbios "of Lesbos," Greek island in northeastern Aegean Sea (the name originally may have meant "wooded"), home of Sappho, great lyric poet whose erotic and romantic verse embraced women as well as men, hence meaning "relating to homosexual relations between women, characterized by erotic interest in other women" (in continuous use from 1890; the noun lesbianism from this sense is attested from 1870) and the noun, which is first recorded 1925.

Sappho's particular association with erotic love between women (with or without concurrent relations with men) dates to at least 1732 in writing in English, though the continuous use of lesbian and the modern words formed from it are from late 19c. The use of lesbian as a noun and an adjective in this sense seems to follow the same pattern.  

In another Place the same commentator conjectures, that Myra is a Corruption of Myrrhina а famous Courtesan of Athens, who first practis'd and taught in that City Sappho's Manner and the Lesbian Gambols. ["Peregrine O Donald" (William King), "The Toast," 1732]

Before this, the principal figurative use of Lesbian was lesbian rule (c. 1600 and especially common in 17c.) a mason's rule of lead, of a type used in ancient times on Lesbos, which could be bent to fit the curves of a molding; hence, figuratively, "pliant morality or judgment."

And this is the nature of the equitable, a correction of law where it is defective owing to its universality. ... For when the thing is indefinite the rule also is indefinite, like the leaden rule used in making the Lesbian moulding; the rule adapts itself to the shape of the stone and is not rigid, and so too the decree is adapted to the facts. [Aristotle, "Nicomachean Ethics"]

It also was used in English from 1775 in reference to wines from Lesbos. Though the specific "pertaining to female homosexuality" is recent, Lesbian had long before that a suggestion of "amatory, erotic," "From the reputed character of the inhabitants and the tone of their poetry" [Century Dictionary]. The island's erotic reputation was ancient; Greek had a verb lesbiazein "to imitate the Lesbians," which implied "sexual initiative and shamelessness" among women (especially fellatio), but not necessarily female homosexuality, and they did not differentiate such things the way we have.

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