What is the origin of the idiom "loan shark" and when to use it?

The idiom "loan shark" has its origins in the criminal underworld and is believed to date back to the early 20th century. It is thought to have originated in the United States where individuals would lend money to borrowers at extremely high interest rates. The phrase is often used to describe illegal lending practices that involve using threats, violence, or intimidation to make borrowers pay back the money they owe.