Noun: A sexual act through which a girl starts giving oral sex to a blindfolded male sexual partner and in the middle of the intercourse, she switches place with a gay/bi man who keeps up suckin'. Then, she removes the blindfold out of the confused guy.
Can also be used as a transitive verb.
Noun: She gave Leonard the good old Guertin.
Verb: Let's Guertin him to the max!