"leading idea, main topic," 1640s, a figurative use (on the notion of "subject often repeated") of the earlier sense "refrain or chorus of a song," 1590s, originally "bass accompaniment to music" (late 14c.), from Old French bordon (Modern French bourdon) "bumble-bee, drone," or directly from Medieval Latin burdonom "drone, drone bass" (source also of Spanish bordon, Portuguese bordão, Italian bordone), of echoic origin.