c. 1400, "to wet, moisten," from Old French moillier "to wet, moisten" (12c., Modern French mouiller), from Vulgar Latin *molliare, from Latin mollis "soft," from PIE root *mel- (1) "soft." Sense of "drudge, labor, toil" (1540s) probably is via the notion of "to labor in dirt or mire." Related: Moiled; moiling.