Origin: 1350–1400; Middle English < Medieval Latin *relentāre, equivalent to Latin re-re- + lentāre to bend, derivative of lentus flexible, viscous, slow
late 14c., "to melt, soften, dissolve," from re-, intensive prefix + L. lentus "slow, viscous, supple" (see lithe). Sense of "become less harsh or cruel" first recorded 1520s. The notion is probably of a hard heart melting with pity.