tribadism (n.)
"lesbian sexual activity," 1811, with -ism + tribade (n.), c.1600, "a lesbian," from French tribade (16c.) or directly from Latin tribas, from Greek tribas, from tribein "to rub, rub down, wear away," from PIE root *tere- (1) "to rub, turn, twist" (see throw (v.)). In reference to a specific sexual technique, from 1965.