A mondegreen (also sometimes spelled "mondagreen") is the mishearing (usually accidental) of a phrase in such a way that it acquires a new meaning.
Me 'n Mrs. G were in Reno today and decided to get some 'strange stuff', by which I mean we went grocery shopping at a brand of supermarket we don't have in our 2-supermarket town.
Mrs. G is a banker and notices banks the way I notice motorcycle shops. As we were turning into the market's parking lot she noticed one.
"Oh, there's a Bursting Appendix Bank" I heard. I mean, I've heard of sperm banks, but...
"Whaaaa...?" I gurgled.
"There's a First Independence Bank. What'd ya think I said?"
She had a good laugh when I told her, and I didn't make this up.