Green’s Dictionary of Slang

muggsy n.

also mugsy

(US) a derog. term of address, i.e. stupid.

[UK]P. Cheyney Don’t Get Me Wrong (1956) 76: That car’s been tailin’ me the whole evenin’ [. ..] Take a look, muggsy.
[Ire]B. Behan ‘Same Again, Please’ After the Wake (1981) 99: Don’t gimme that, mugsy.