1297, from L.
latrina, contraction of
lavatrina "washbasin, washroom," from
lavatus, pp. of
lavare "to wash" (see
lave) +
-trina, suffix denoting "workplace." Its reappearance in 1642 is probably a re-borrowing from Fr.; esp. of a privy of a camp, barracks, college, hospital, etc.
Latrine rumor "baseless gossip" (of the kind that spreads in conversations in latrines) is military slang, first recorded 1918.