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1590s, "to make deaf," from deaf + -en (1). The earlier verb was simply deaf (mid-15c.). For "to become deaf, to grow deaf," Old English had adeafian (intransitive), which survived into Middle English as deave but then took on a transitive sense from mid-14c. and sank from use except in dialects (where it mostly has transitive and figurative senses), leaving English without an intransitive verb here.
deafen deaf·en (děf'ən)
v. deaf·ened, deaf·en·ing, deaf·ens
To make deaf, especially momentarily by a loud noise.