Final answer:
The prefix meaning 'out of' or referring informally to a former spouse is 'Ex-'. It denotes a past condition or extraction from something.
Step-by-step explanation:
The prefix that means "out of," or is informally used to refer to a former spouse, is Ex-. This prefix is attached to the beginning of a word to indicate a former status or to specify that something is coming out from something else. For example, exit means to go out of a place, and ex-husband or ex-wife refers to a former spouse.