Longing is the verb form of taking a long position, a topic covered in full under long and short positions. This page explains how the word is used in crypto trading chat.
When a trader says they are "longing bitcoin at 60k", they usually mean they are opening a position that profits if the price rises from that level. In crypto, the phrase most often refers to a leveraged trade on perpetual futures or margin, not to simply buying and holding coins, although it can mean either. Its counterpart is "shorting", betting on a fall.
The distinction matters because a leveraged long behaves very differently from an outright purchase. With leverage, a modest price drop can trigger forced liquidation and wipe out the trader's collateral, while a coin bought outright can fall and recover without being sold. Holding a leveraged long on perpetual futures also involves paying or receiving a funding rate, which is usually paid by longs when the market is crowded on the buy side.
The slang appears constantly on social media and trading forums, often alongside screenshots of open positions. Statements that someone is "longing" a coin describe their trade, not evidence that the price will rise, and data on the balance between longs and shorts across exchanges is sometimes used as a rough gauge of market mood.