"When I use a word . . . it means just what I choose it to mean—neither more nor less."
--- Humpty Dumpty
What does the phrase "paramour in waiting" mean?
As I am confident the reader already knows, a paramour is an illicit lover.
But the meaning of the phrase "paramour in waiting" depends very much on who is using it . . .
[note: 'paramour' is gender neutral, it can be applied to both men an women equally. But to simplify this discussion, we will suppose we have a male with a female wife and a female potential lover (paramour).]
. . . So who is using the phrase?
The Paramour herself? "I am a paramour in waiting."
if so is she bragging? angry? sad? etc. The meaning will obviously change accordingly.
The man? "She is my paramour in waiting."
if so, what is his intention? To show pride? relief? confusion? etc.
The wife? "She is my husband's paramour in waiting." Is the wife angry? happy? resigned? etc.
What if the phrase was spoken by a third party? neutral? supportive? hostile?
etc. etc. etc. etc. etc.
In the end, the phrase, 'paramour in waiting,' means what the speaker or writer chooses it to mean == 'neither more nor less.'