This is short for Lamborghini.
It is a term used by those who engage in market speculation. This can apply to stocks but became especially popular for cryptocurrency. It is the dreams of massive wealth generation through price appreciation of assets held.
The idea that one is going to enjoy massive gains. The attitude becomes prevalent during the euphoria that is common in bull markets. People think whatever asset they are holding is going to moon, providing them with enormous returns.
Financial Freedom
Market participants look for the opportunity to make so much money that the price of an expensive automobile is of no concern. What do newly minted millionaires buy? A Lamborghini.
This is the concept of "Lambo" which is also tied to mooning, the idea that prices will skyrocket.
Much of this stems from the early Bitcoin days. There were a number of millionaires created from the early buys (or mining) of BTC when the price was low. Being a HODLer meant portfolios garnered outsized returns that produced generational wealth.
The coins in Bitcoin wallets were not the only ones to explode. Altcoins also were able to join the party during bull runs. Many holding ETH were able to realize massive price appreciation.
Bubble
Market excess always seems to take over. The result is usually a bubble. When talk of mooning and Lambos becomes commonplace, it is a sign that markets moved to far.
Throughout the history of cryptocurrency, there were severe pullbacks. Bear markets often mean a drop of more than 70% in Bitcoin and 90%+ in altcoins. This drawdown can be painful causing the strong ones to HODL.
The popping of bubbles is a natural part of market movements. When the prices move too far, reversals happen The clearing out continues until there are few left to sell. It is at the nadir when another bull can start.