To look at someone through a mirror. Preferrably that person be showering. This refers to the famous substitute teacher/little league baseball coach, Doc Fuss. Doc Fuss was famous for watching young kids shower after gym class by looking at the children through mirrors.
Bob: You're mom gay
Steve: Your dadlesbian Bob: Your granny tranny
Steve: Your grandpap a trap
Bob: Your brother a mother
Steve: Your niece obese
Bob: Your family tree LGBT
Steve: Don't make me do it.
Bob: Do it!
Steve: Your cousin be fussin
(Bob then explodes, barely leaving Steve alive)