Joking, teasing, or making fun of someone in a lighthearted way; or the act of separating chaff from grain.
From Old English ceaf (chaff) with the -ing suffix. The teasing meaning comes from the idea of 'separating wheat from chaff' in conversation—filtering out the serious from the joking.
Chaffing as teasing comes from literally separating chaff from wheat—the same metaphor of 'removing the worthless bits' applies whether you're threshing grain or roasting your friend!
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