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 dad lesbian 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)
Fusking is simply using a program to extract files names from a website that would seem obvious. Like 1.jpg, 2.jpg, etc.
For the record: These other definitions aren't "fusking" wrong, they're fucking wrong. If you're typing "s" where your "c" is... consider a keyboarding class, n00b.
Amber: Oh hey Nikki, what are you gonna do tonight?
Nikki: I'm gonna try fusking in our friend's privatephotobucket for pictures they wouldn't want us to see.