A person whose investment in crypto falls to zero. In the mid-2000s, a Romanian programmer using the handle "Lord Ashdrake" shorted Bitcoin (BTC) and lost everything. Previously, he made a lot of money shorting the crypto.
Like the stock market world, some crypto exchanges support margin accounts and shorting, which means borrowing assets and immediately selling them in order to buy them at a lower rate later and pay back the debt. See
crypto glossary.