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.<sup>[1]</sup>

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