Quick and energetic; also fashionable and stylish, or irritable and bad-tempered.
From 'snap,' likely from Middle Dutch 'snappen' (to snap or grab), 'snappy' developed multiple meanings reflecting quick, sharp actions or attitudes.
The phrase 'make it snappy' means hurry up—and it became popular in 1930s-40s slang, which is why you hear it in old movies and it sounds charmingly vintage now!
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