A malapropism (also called a Dogberryism or acyrologia) is the substitution of a word for a word with a similar sound, in which the resulting phrase makes no sense but often creates a comic effect.
Elaboration here: http://en.wikipedia.org/wiki/Malapropism
We've all heard them. We've all made them. Let's share. I'll start:
Bagman has a co-worker I've dubbed Mr. Malaprop, because he seems to come out with a new one every day. Recent ones I enjoyed were "cutting-age technology" (that's plastic surgery, right?) and "unthaw" (I guess that means freeze).
Heard on a radio talk show -- the caller was lamenting the fact that our society is quick to sue: "We've become too liturgical!"
A friend reflecting on a period when he didn't want to be around people: "I just wasn't feeling socialistic."
Someone telling me a particular action I took was taken too soon: "That was immature!"
I'm guilty, too. I once referred to a particular surgery as a "tubal litigation."