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1932, alteration of dialectal verb smouch "to kiss" (1570s), possibly imitative of the sound of kissing (cf. German dialectal schmutzen "to kiss"). An earlier alteration produced smudge (v.) "to kiss, caress" (1844). Related: Smooched; smooching. As a noun by 1942.
Incontestable evidence; the GOODS: They had discovered the ''smoking gun'' that would destroy the general's case/ In fact, there may be no ''smoking gun,'' no incontrovertible, black-and-white evidence of wrongdoing
[1970s+; fr the image of a murderer caught with the fatal smoking firearm still in hand]