Fuckshit is anything that you know you shouldn't be doing. It also is used to describe anything out of the ordinary or illegal. It is a generalization of all things that make you second guess doing it.
Sharon be on some fuckshit not paying TJ the money she owes. If she wasn't going to cover it, then why did she vouch for Jill and tell him she'd pay it?
Buck is locked up because Mark got his ass tangled up in some fuckshit and they got caught.
You are walking down the street and pull your wallet out of your pocket. Your car keys come out too and fall down into a sewer grate where you will never recover them. You say fuckshitpiss.