Why Does Plum Mean Completely?

The word plum is seldom being used to convey the idea of something being completely done but one of its uses can relate to the phenomenon to some extent. The word plum is used as an adjective to convey the idea of something that is extremely desirable and people crave to achieve that. If something is so much desirable it can be considered as being complete or perfect. So this use of the term might have encouraged people to use the term this way.
     
Though the term is seldom being used this way, it has several other uses and mainly the word conveys the idea of a fruit that is soft and round and has purple or red skin. The word can also be used to signify a colour that is dark reddish-purple.