late 14c., "pertaining to (sexual) reproduction," in membres genytal "the genitals," from Latin genitalis "pertaining to generation or birth; fruitful" (also a by-name of the goddess Diana), from genitus, past participle of gignere "to beget" (from PIE root *gene- "give birth, beget"). Hence the English word came to mean "pertaining to the organs of generation." As a noun meaning "sex organ" from mid-15c. (plural genitals is from late 14c.).
updated on April 25, 2017