Green’s Dictionary of Slang

what can I do you for? phr.

[do v.1 (2a)]

a facetious reversal of SE what can I do for you?

[UK]Partridge DSUE (8th edn) 1322/1: since early 1920s.
P. Blauner Casino Moon 136: He was reading a copy of the Wall Street Journal when I walked in [...] ‘What can I do you for?" he asked with a sniff .
[US]J. Ridley Everybody Smokes in Hell 46: Now, what can I do you for? Want Coppola’s new flick?