1650s, probably from a noun use of Middle English molsh (adj.) "soft, moist" (early 15c.), from Old English melsc, milisc "mellow, sweet," from Proto-Germanic *mil-sk- (cf. Dutch mals "soft, ripe," Old High German molawen "to become soft," German mollig "soft"), from PIE root *mel- "soft" (see mild).
1802, from mulch (n.). Related: Mulched; mulching.