Hit the bid

In a two-way price the bid is what a buyer will pay and the ask, also called the offer, is what a seller will accept. Selling without waiting means dealing at the bid, and the trade is described as hitting it. The mirror phrase for buying without waiting is lifting the offer, or paying the offer. Both are descriptions of which side of the quote the deal happened on, and neither says anything about size or about intent.

The phrase identifies who crossed the spread. The party that hits the bid is the aggressor and pays the bid-ask spread; the party that had posted the bid is passive and is paid it. A market order to sell hits the bid by construction, since it asks for the best price currently available rather than naming one. On a trading platform the sell control deals at the bid and the buy control at the ask, which is why a position opened and closed in the same breath shows the distance between the two as a loss before price has moved at all.

The point most often missed is that a bid is quoted with a size attached, and the phrase describes the price convention rather than a guarantee about the whole order. An order larger than the size showing at the best bid deals partly at that level and partly at worse levels further into the book, so the average price obtained is below the bid that was quoted. That difference is slippage, and it grows with order size and with how thin the book is at the moment of dealing.