Definition of 'Loan Shark'
A person or entity that charges borrowers interest above an established legal rate. Depending on where a person lives, lenders typically cannot charge more than 60% interest per annum. A loan shark, then, would be someone who illegally charged interest over the state's legal limit, which could range up to, or even over, 100%.
Investopedia explains 'Loan Shark'
For example, a loan shark would lend $10,000 to a person with the provision that they be repaid $20,000 within 30 days.
A big word of caution is that loan sharks will often back their lendings with threats of violence or damage to a person's reputation as a way to ensure the loans are repaid. If you find yourself in a position of owing to a loan shark, make sure you talk to a financial (and maybe a legal) professional to help you get out of the situation.