1. [derogatory] To imperfectly transform information.
2. A comprehensive rewrite of a routine, data structure or the whole program.
3. To modify data in some way the speaker doesn't need to go into right now or cannot describe succinctly (compare mumble).
4. To add spamblock to an email address.
This term is often confused with mung, which probably was derived from it. However, it also appears the word munge was in common use in Scotland in the 1940s, and in Yorkshire in the 1950s, as a verb, meaning to munch up into a masticated mess, and as a noun, meaning the result of munging something up (the parallel with the kluge/kludge pair is amusing). The OED reports “munge” as an archaic verb meaning “to wipe (a person's nose)”.