mod. silly; crazy. : Kelly was acting daffy because she was so happy.
Dictionary of American Slang and Colloquial Expressions by Richard A. Spears.Fourth Edition. Copyright 2007. Published by McGraw Hill.
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Word Origin & History
daffy
1884, perhaps from daft (q.v.), or from obs. daffe "halfwit" (cf. 15c. daffish), of unknown origin.