Similarily to stonks, you use it to express a financial decision that resulted in a lot of financial gain. Mostly in a short amount of time. In this case, it is most often used not ironically. You may also say it just for fun.
A: "I sold my 15 year old car for 3000$ yesterday. Also, I got a raise from my boss today! I'm making so much money right now!"
B: "BIG STONKS ONLY!!"
B: "BIG STONKS ONLY!!"
by Sheeshsquatch69 June 10, 2020