I suspect "Monigan" should have been "Mondegreen." Mondegreen is a term that has been explained and discussed many times in Mudcat. Click here for an example.

So I think the writer is trying to say that the word was originally something else, and was changed to "pussy" because someone misheard it.

I'm not familiar with the song, so I can't guess what word the writer had in mind, but you might be able to figure it out.