What does no pun intended mean?

No pun intended” is a common expression used to highlight a pun while ostensibly claiming that the pun was unintentional. This phrase is often used humorously to call attention to a pun that the speaker recognizes is cliché and likely to elicit a groan. However, in some cases the phrase is used sincerely, indicating that the speaker did not notice the pun until after it was spoken.