Green’s Dictionary of Slang

smush n.

[? mush n.2 ]

(Irish/US) the mouth.

[Ire]P.W. Joyce Eng. As We Speak It In Ireland (1979) 329: Smush, used contemptuously for the mouth.
[US]D. Runyon ‘Breach of Promise’ in Runyon on Broadway (1954) 27: He [...] kisses her ker-plump right on the smush.