people who have made a major loss in the stock market or those who are holding on to shares that have fallen a lot in value [colloquial]
Definitions (粵典–words.hk)
名詞
買咗股票後，因股票價格下跌而未能套現嘅人 (literally: crab people)People who bought stock which have fallen in value and unwilling to sell the stock.
佢入咗10手5號仔後，個市就跌到阿媽都唔認後，變咗蟹民
佢入咗10手5号仔后，个市就跌到阿妈都唔认后，变咗蟹民
The market dropped quite a lot after he bought 10 lots of no. 5. Now he doesn't want to sell it.