same-sex adj.

"identical, equal; unchanging; one in substance or general character," from Proto-Germanic *samaz "same" (source also of Old Saxon, Old High German, Gothic sama, Old High German samant, German samt "together, with," Gothic samana "together," Dutch zamelen "to collect," German zusammen "together"), from PIE *samos "same," from suffixed form of root *sem- (1) "one; as one, together with."

Old English seems to have lost the adjective except in the adverbial phrase swa same "the same as" (literally "so same"). But the word that emerged in Middle English as "the ordinary adjectival pronominal designation of identity" [OED] is considered to be more likely (or mostly) from the Old Norse cognate same, samr "same." In its revival it replaced synonymous ilk.

As a pronoun, "the person or thing just mentioned," from c. 1300. In Middle English also a verb and an adjective, "together, mutually" (as in comen same "gather together, unite," kissen same "embrace one another").

Colloquial phrase same here "the same thing applies to me" as an exclamation of agreement is from 1895. All the same is from 1803 as "nevertheless, in spite of what has been mentioned." Same difference, a curious way to say "not different; equal," is attested from 1945. Often expanded for emphasis: ilk-same (mid-13c.); the self-same (early 15c.); one and the same is in Wycliffe (late 14c.), translating Latin unus atque idem.

late 14c., "males or females considered collectively," from Latin sexus "a sex, state of being either male or female, gender," a word of uncertain origin. "Commonly taken with seco as division or 'half' of the race" [Tucker], which would connect it to secare "to divide or cut" (see section (n.)).

Secus seems the more original formation, but it is strange that the older texts only know sexus. The modern meaning of sectiō 'division' suggests that sec/xus might derive from secāre 'to sever', but the morphology remains unclear: does sexus go back to an s-present *sek-s- 'to cut up', or was it derived from a form *sek-s- of the putative s-stem underlying secus? [Michiel de Vaan, "Etymological Dictionary of Latin and the other Italic Languages," Leiden, 2008]

The meaning "quality or character of being either male or female" with reference to animals is recorded by 1520s; by 19c. this meant especially "the anatomical distinction between male and female as evidenced by physical characteristics of their genital organs and the part taken by each in reproduction." Extended by 1560s to characteristics or structures in plants which correspond to sex in animals.

It is curious that the Anglo-Saxon language seems to have had no abstract term for sex, which was expressed only severally as manhood or womanhood. [Thomas Wright, note to "Anglo-Saxon and Old English Vocabularies," 1884] 

Also especially the sex "the female sex, womankind" (1580s). The meaning "sexual intercourse" (have sex) is by 1906; the meaning "genitalia" is suggested by 1933 ("Fumes of Formation") and probably is older. Sex symbol by 1871 in anthropology; the first person to whom the term was applied seems to have been Marilyn Monroe (1959). Sex-kitten is attested by 1954 (Brigitte Bardot). Sex object is by 1901, originally in psychology; sex appeal is attested by 1904.

For the raw sex appeal of the burlesque "shows" there is no defense, either. These "shows" should be under official supervision, at the least, and boys beneath the age of eighteen forbidden, perhaps, to attend their performance, just as we forbid the sale of liquors to minors. [Walter Prichard Eaton, "At the New Theatre and Others: The American Stage, Its Problems and Performances," Boston, 1910]

Sex-life is attested by 1887. Sex-drive is by 1916 (sex-impulse by 1911). Sex-education is by 1894; sex therapist is by 1969, in early use often in reference to Masters and Johnson. Sex-crime is by 1907; sex-maniac by 1895; sex-fiend by 1931 (in a New York Daily News headline).