Hard-to-Borrow (HTB)
A stock that's difficult to short - often because supply is tight or the stock is heavily shorted already.
Brokers charge an additional daily fee to borrow hard-to-borrow shares (can be 10%+ annualised). Sometimes shorting is unavailable entirely. A stock becoming HTB is often a sign of squeeze risk.
Learn to actually use Hard-to-Borrow.
Definitions are the easy part. The free first five modules put this on a real chart and make you do the work. No card required.