Green’s Dictionary of Slang

(US) in disguise.

1883		G.W. Peck Peck’s Bad Boy and His Pa (1893) 263: You would pass in a colored prayer meeting, and no one would know you were galvanized.
1939		(con. 1914) D. Coolidge Old Calif. Cowboys 65: He’s [i.e. an American] a galvanized Mex [HDAS].