A Yiddish insult meaning a thief, con man, or other such unscrupulous and untrustworthy character looking to conduct a scam, ripoff, or similar evil and dishonest method of personal gain.
The word is an insult. A ganef wouldn't generally call himself a ganef, no matter how proud of himself he was, but he might be angrily called a ganef by his victim.
Bernie Madoff convinced me to put my life's savings into his ponzi scheme. That no-good ganef cost me everything!