A phrase, saying or sentence that can be interpreted in two different ways. One of which is usually dirty.
An unintentional double entendre that happened to me at work
Me working at store: Do you want me to put that in a bag?
Female customer: Yes please.
Me: Do you want a regular sized one or a huge one?
Customer: I want a huge one, do you have a huge one?
Me: Oh, I've got a huge one alright.