Green’s Dictionary of Slang

(US) one’s signature.

1965–70		in DARE n.p.: Joking words . . for a name signed to a paper: ‘I’ll put my on that.’) [...] John Esquire [...] John Handle [...] John Rogers [...] John Smith [...] John Willy [...] John Brown.
2005		J. Stahl I, Fatty 138: Before the guy even gets my John Q and turns away, Daddy shakes his head and says [etc.].