putting on a raincoat
wearing a condom during intercourse
He does not want gonorrhea, so he is putting on a raincoat.
Put a raincoat on it
An old way of saying put on a condom if you don't want to impregnate someone.
John: Jack, is...is that YOUR baby your holding?!
Jack: Yeah
John: You didn't put a raincoat on it, did you?
Jack: No
Jack: Yeah
John: You didn't put a raincoat on it, did you?
Jack: No