I really don't like that term, money you can afford to lose, I will prefer we use the term discretionary income, when we use the term money we can afford to lose when it comes to bitcoin investment it sounds like gambling, and using that term may create some mindset that Bitcoin is like gambling to a newbie who wants to go into Bitcoin investment.
For a long term Bitcoin investor his using his discretionary income and not money he can afford to lose.
I think you are misunderstanding or misinterpreting the phrase "investing money you can afford to lose". Let's forget about Bitcoin investment for a moment and consider real estate investment. Can you guarantee that real estate (business) investment will be profitable or you will not lose? If there is a possibility of losing here, can you call it gambling? Investing means there is a possibility of losing, so saying that you consider it gambling is wrong. I think you are not very familiar with the definition of gambling. Gambling is a short-term game of chance, where you have to rely on luck to win and the possibility of losing is high.
Investing money you can afford to lose in the context of Bitcoin investment means that you do not need that money or that if you lose that money, it will not have any negative impact such as disrupting your daily needs or changing your quality of life. We would never want to lose that money, but we must be mentally capable of losing. Since Bitcoin is reliable and trustworthy, you should never blindly trust it. You need to be prepared for small potential outcomes and have the ability to take action.
There is a big difference between discretionary spending and money you can afford to lose, which you may be confused about. Discretionary spending does not mean unnecessary or excess money. Any money left over after meeting your daily needs that you will never need or are prepared to lose is considered investmentable or excess money. Your source of income is not the main issue here.