An animal or person who is sly, sneaky, and opportunistic. Someone who is looking to slip their way into a situation and make it shittier.
"Your friend is such a shitweasel, every time he comes over, he creates drama by asking questions and exploiting their answers to upset people."
"My cat is the ultimate shitweasel. He somehow manages to open the door to my bedroom and urinate in the corner, even though I am very vigilant about keeping it closed and locked."