Soften

wiktionary

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From Middle English softenen, softnen, equivalent to soft +‎ -en.

etymonline

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soften (v.)

late 14c., "to mitigate, diminish" (transitive), from soft (adj.) + -en (1). Meaning "to make physically soft" is from 1520s; intransitive sense of "to become softer" is attested from 1610s. Soften up in military sense of "weaken defenses" is from 1940. Related: Softened; softening.