Calling someone “shameless” is a little too literal, and when you want to throw in indirect insults, you need to be a little more creative.
In comes this saying that says someone’s face is so large, it’s bigger than their back. Basically, they don’t try to hide their face after doing something wrong.
Ironically, another way to call someone shameless in Spanish is to say they’re “faceless.” Darned if you do, darned if you don’t.