I'm surprised that nobody has mentioned that the phrase "pun intended" is a form of wordplay itself:
As others have commented, a pun is a play on words, usually using words that sound the same or similar to give multiple meanings to a phrase.
Sometimes when writing or speaking it is hard or unavoidable to use a phrase that creates such a double meaning - potentially obscuring the speaker's point and/or making it seem like they are joking. In this situation it is common for people to say
(No pun intended)
This, I believe, is the original phrase.
Playing on this, some people occasionally reverse the phrase by removing the negative:
to make it clear to the listener that yes, they are using wordplay to make their point.
(Interestingly Google has ~11 million hits for "pun intended", but ~21 million for no pun intended, even though every page that contains "no pun intended" must contain "pun intended"!)