An individual whose partner commits infidelity and does not hide the signs. The individual, however, does not leave the relationship but instead tries to please their partner, who in turn ridicules him/her.
A cunumunu is no ordinary idiot, simpleton, or stupid person, but an individual who is getting "butt","horned", and is aware of it, because his or her partner does not  conceal obvious signs of infidelity. The cunumunu does not react like most and break with the partner, but takes increasing amounts of ridicule from the mate, often going out of the way to please that person, and seems powerless to extract him/herself from the relationship.
The meanings of words seem to evolve and change with the passage of time, but this was the accepted definition in Trinidad up to the late 1960's. The Mighty Sparrow sang about a cunumunu in, I think the name of the song was 'Take yuh bundle and leave and go', early 60's.