Meaning of A MOUSE IN HIS POCKET in English

A MOUSE IN HIS POCKET

Phrase used to describe someone large, probably very strong, but intensely stupid. From _Of Mice and Men_[?] Example: We've got a new guy at work who worries me; I swear I think he's got a mouse in his pocket.

Slang English vocab.      Английский сленговый словарь.