It is an extremely rude word that should never be used in front of a lady, unless said lady is a classless whore, or you really, really want to.
It cannot be used as an insult, but it can be used to make someone feel insulted.
Com aquelas calças justas, vê-se-lhe a rata toda! He He He! - Those super tight pants give her a camel-toe. Heh heh heh!