The word 'sex' entered English in the fourteenth century from Old French 'sexe,' itself from Latin 'sexus,' meaning the state of being male or female. The Latin word is most likely derived from 'secāre' (to cut, to divide), from the Proto-Indo-European root *sek- (to cut). The semantic logic is straightforward: sex is the great division, the cutting of a species into two complementary halves.
This etymology connects 'sex' to a surprisingly large family of English words. 'Section' (a cut part), 'sector' (a cutter; a cut division), 'dissect' (to cut apart), 'bisect' (to cut in two), 'secant' (a cutting line), and 'sickle' (a cutting tool) all descend from the same root. Most remarkably, 'insect' belongs to this family: it comes from Latin 'insectum,' the past participle of 'insecāre' (to cut into), which is itself a calque (loan-translation) of Greek 'entomon' (ἔντομον), meaning 'cut into' — referring to the segmented bodies of insects. The study of insects, 'entomology,' preserves the Greek version of the same metaphor
In its original English usage, 'sex' meant exclusively the biological category — male or female. The word appears in Wycliffe's Bible translation of 1382, and for centuries it carried no connotation of sexual activity. When people in the sixteenth or seventeenth century wrote about 'the fair sex,' they meant women as a category, with no erotic undertone.
The use of 'sex' to mean sexual intercourse is much more recent than most people assume. The earliest clear attestation in this sense dates to 1929, in D.H. Lawrence's poem 'Pansies.' Before that, English used various euphemisms and circumlocutions. The twentieth century saw a rapid expansion: 'sex appeal' (1904), 'sex symbol' (1911), 'sex drive' (1918), and eventually 'sexy' (1923, originally meaning 'concerned with sex,' the modern sense of 'attractive' dating from the 1950s).
The Latin word 'sexus' should not be confused with 'sex' meaning 'six' (Latin 'sex,' from PIE *sweks), despite the identical spelling in Latin. The two words are unrelated. This has not prevented folk etymologies from attempting to connect them — the most persistent being the false claim that 'sex' relates to the six days of creation.
The adjective 'sexual' appeared in English around 1651, borrowed from Late Latin 'sexuālis.' 'Sexuality' followed in 1789, 'sexism' in 1968 (modeled on 'racism'), and 'sexist' shortly after. The prefix 'bi-' was combined with 'sexual' to form 'bisexual' in 1824, originally in botany (describing flowers with both male and female organs) before being applied to human sexuality in the twentieth century.
The distinction between 'sex' (biological) and 'gender' (social/cultural) was articulated by the sexologist John Money in 1955 and popularized in academic discourse through the 1970s and 1980s. Before Money's distinction, 'gender' was primarily a grammatical term (masculine, feminine, neuter), and 'sex' covered both biological and social aspects of the male-female divide.