meld-
“to soften, to melt”melt, soften
PIE root meaning to soften or to melt. Source of English "melt," "mild," "smelt," and words for softening by heat.
Discussion
*meld- is a Proto-Indo-European root meaning "to soften" or "to melt," yielding vocabulary for the transformation of solid to liquid through heat.
The root gives Old English meltan "to melt" (modern English melt) and milde "mild, gentle" (the semantic connection being "softened"). German mild and schmelzen "to melt" continue both branches. The causative form *mold-eye- gives English smelt (to melt ore), with the s-mobile prefix.
Latin mollis "soft" (whence English mollify, emollient, mollusk) derives from a form *mld-wi-. Greek meldein "to melt" preserves the root directly. Sanskrit mr̥du- "soft, tender" shows the expected Indo-Iranian development.
In Slavic, Old Church Slavonic mladŭ "young, tender" extends the "soft" sense to youth—things that are young are soft. This semantic extension from physical softness to youth and gentleness runs through multiple branches.
Modern descendants include English melt, mild, smelt, and through Latin: mollify, emollient, mollusk ("soft one"), and mulch (possibly).
Notes
Source of English "melt", "mild", Greek "meldein". Zero-grade *ml̥d-.