It is commonly used, so it's not really a slip of the tongue. It probably originates in a playful inversion of the usual order for words, which would be “What can I do for you?”.

Also, beware! Unlike what I thought the first time it was said to me, this is not commonly meant (and should not be understood as) a sexual propoposition.^[1]

[1] See: do (transitive verb; vulgar; slang) • have sexual intercourse with.

It is commonly used, so it's not really a slip of the tongue. It probably originates in a playful inversion of the usual order for words, which would be “What can I do for you?”.

Also, beware! Unlike what I thought the first time it was said to me, this is not commonly meant (and should not be understood as) a sexual proposition.^[1]

[1] See: do (transitive verb; vulgar; slang) • have sexual intercourse with.