To make rough or uneven; to irritate or aggravate; to embitter or annoy.
From Latin 'exasperare', from 'ex-' (thoroughly) + 'asper' (rough); the root is also found in 'asperity,' both describing roughness becoming metaphorically applied to emotions.
The word 'exasper' literally means 'to make rough,' but medieval speakers realized that feelings can get rough just like surfaces—when someone won't stop bothering you, they've 'roughed up' your mood.
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