If you're wondering why that act qualifies as Fiona's greatest misdeed on Shameless, you may not remember everything that went down between Fiona and Gus.
As a quick refresher, Gus turned up on season 5 of Shameless, and no sooner had the kind-hearted bass player entered Fiona's orbit, he'd gone and married her. In the process, he offered his bride his grandmother's ring, which the departed woman had somehow managed to hang on to while being held in a concentration camp during World War II. While the antique ring obviously had clear intrinsic value, the sentimental value to Gus and his family no doubt made it priceless.
In case it wasn't clear, Gus may have been the most caring guy Fiona ever coupled with on Shameless – which, of course, made him the definition of boring in Fiona's book. Not surprisingly, the happy couple's nuptials didn't last long. Fiona cheated on Gus with her sleazy ex Jimmy (Justin Chatwin), and eventually had a tryst with her in-recovery boss Sean (Dermot Mulroney). By the time Gus and Fiona went to part ways after all that had happened, the wedding ring became a point of contest between the two. The biggest problem? Fiona couldn't return the ring to Gus in a timely manner because, well, she'd already pawned it.
Gus later had a small amount of revenge when he and Fiona were going through their nasty divorce, essentially holding Fiona's house and life hostage until he got the ring back. In one of Shameless' more, ahem, shameless narrative twists, Fiona had to turn to Sean to get the money to buy the ring back from the pawn shop. And while Sean was returning the ring to Fiona, he used it to propose to her himself... with Gus watching.
Now, don't get us wrong — Sean was a much better fit for Fiona, at least for a short while, but she really went above and beyond in tormenting poor Gus.
