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early 14c., from Old Norse væla "to lament," from væ "woe" (see woe). Of jazz musicians, "to play very well," attested from 1955, American English slang (wailing "excellent" is attested from 1954). Related: Wailed.
c.1400; see wail (v.).
[fr the notion of a well-performed blues number, with its melodious lamentations]