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Phrase(s): a slippery customer
1. Fig. a clever and deceitful customer. • Watch out for that guy with the big padded coat. He may snatch something. He’s a real slippery customer. 2. Fig. a slippery creature. • This little fish is a slippery customer. Get me something to scoop it back into its bowl.
McGrawhill's American Idioms And Phrasal Verbs