To remove fleas from an animal, or to treat an animal or place to eliminate fleas.
From de- (remove, strip of) and flea. This is a straightforward compound formation where de- acts as a reversal prefix, similar to how 'defrost' means to remove frost. Entered English in modern times as pet care became common.
Your vet might say 'we need to deflea your dog' instead of the wordier 'remove all the fleas from your dog'—it's a perfect example of how English loves to prefix 'de-' to turn any problem into a solution!
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