1640s, "continue to advance," also "manage, be engaged in," from carry (v.) + on (adv.). The meaning "conduct oneself in a wild and thoughtless manner" is by 1828. Carryings-on is from 1660s as "questionable doings," from 1866 as "riotous behavior." As an adjective, carry-on, in reference to luggage that may be brought into the passenger compartment of an airliner, is attested by 1965.
"excuse for being," 1864, first recorded in letter of J.S. Mill, from French raison d'être, literally "rational grounds for existence."
also g-spot, 1981, short for Gräfenberg spot, named for German gynecologist Ernst Gräfenberg (1881-1957), who described it in 1950.