To make unnecessary complaints or criticisms about minor or unimportant things.
From Latin 'cavillari' meaning 'to jest' or 'to mock,' possibly related to 'cavilla' meaning 'mockery.' The sense evolved from joking criticism to petty fault-finding over time.
Cavil comes from the same root as 'caveat'—both involve being doubtful or critical, but caveat warns you beforehand while cavil complains about what's already happened!
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