Brewer's: Robin Mutton
(A). A simpleton.
“Do you see this ram? His name is Robin. Here, Robin, Robin, Robin. ... We will get a pair of scales, and then you, Robin Mutton [Panurge], shall be weighed against Tup Robin, ... etc.” —
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894