1 The selling of securities, commodities, etc., by a broker because the original buyer is unable to pay for them. This invariably happens after a fall in market price (the buyer would have taken them up and sold them at a profit if the market had risen). The broker sells them at the best price available and the original buyer is responsible for any difference between the price realized and the original selling price, plus the costs of selling out. Compare buying in.
2 Closing a trading position held for a client because a margin call has not been met.
3 Selling the whole of a new issue of a security.