1)A shady or duplicitous person. A confidence trickster or scammer. A member of a group liable to defraud others - for instance, solicitor, estate agent, bank manager etc.
2)Someone who seems to thrive in an unpleasant environment and always seems to be at the heart of any problems being created.
3)Some it is difficult to prove guilty of fraudulent, unsavoury or similar acts.
Imagine a pile of excrement, with a slippery weazel popping up, looking left and right while standing up with the excrement on this head, like a proud hat, and then disappearing down again.
"I had a call from Natwest again. They are trying to sell me PPI but its clear I'll never be able to make a claim. They never get done for fraud. They are such shitweazels".
Steven walked in to the Gents toilets looked through all the cubicles to try and find a shiteable enough toilet. The rest of the cubicles were kinda messy but he found at least one shiteable toliet.
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."