Someone who does very dodgy things, rips you off, acts shonky in general, etc. A consistent shonkster is someone to be avoided, as being in their presence will most likely have bad implications for you.
Can be applied to anyone at all who shonks you over.
"Hey, I got charged way too much for this by that guy. What a shonkster."
"Hugh is becoming a major shonkster, I'd watch out."