Where does “Runyonese” come from?

Runyonese (English) comes from English ese, from Mexican Spanish ese, from Latin ipse, from Proto-Italic *e(s)peso.

Ancestry of “Runyonese”, step by step

Step	Language	Word	Meaning
1	English	ese	dude, man
2	Mexican Spanish	ese	The name of the Latin-script letter S; that;...
3	Latin	ipse	himself, herself, itself, the very, the actual;...
4	Proto-Italic	*e(s)peso	—

Runyonese — Etymology, Origin & Word History | Etymology Explorer