Etymology

sex (n.)

late 14c., "males or females collectively," from Latin sexus "a sex, state of being either male or female, gender," 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]

Meaning "quality of being male or female" first recorded 1520s. Meaning "sexual intercourse" is attested by 1906; the meaning "genitalia" is attested by 1938. 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 drive is by 1918; sex object by 1901; sex symbol by 1871 in anthropology; the first person to whom the term was applied seems to have been Marilyn Monroe (1959). Sex therapist is from 1974.

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]

Origin and meaning of sex

sex (v.)

1884, "to determine the sex of," from sex (n.); to sex (something) up "increase the sex appeal of" is recorded from 1942. Related: Sexed; sexing.

Definitions of sex
1
sex (n.)
activities associated with sexual intercourse;
they had sex in the back seat
Synonyms: sexual activity / sexual practice / sex activity
sex (n.)
either of the two categories (male or female) into which most organisms are divided;
sex (n.)
all of the feelings resulting from the urge to gratify sexual impulses;
he wanted a better sex life
the film contained no sex or violence
Synonyms: sexual urge
sex (n.)
the properties that distinguish organisms on the basis of their reproductive roles;
she didn't want to know the sex of the foetus
Synonyms: gender / sexuality
2
sex (v.)
stimulate sexually;
Synonyms: arouse / excite / turn on / wind up
sex (v.)
tell the sex (of young chickens);
From wordnet.princeton.edu