For the sake thoroughness I would point out plum(b) can mean 'utterly' or 'totally' in pretty much any situation in British slang (e.g. 'We're plum out of honey'). A common usage of plum (without the 'b') is as an especially desirable position, assignment, or reward (e.g. an ambassadorship granted as a political plum.); not sure how that one came came about though.