"What can I do you for?" is a colloquial phrase that is sometimes used instead of "What can I do for you?". While the two phrases are similar in meaning, "What can I do you for?" is considered a joke and is not grammatically correct. The phrase is often used humorously to ask someone how you can help them, especially when you are trying to sell them something. It is important to note that "What can I do you for?" is not appropriate in formal or business situations.