All in Definition 1 of 4Pro Tip 1/3Fatigue Expression Use 'all in' when you want to express you've used up all your energy and can't do more. After the marathon, she felt all in. Pro Tip 2/3Colloquial Usage It's a casual phrase, so avoid using 'all in' in formal contexts like business reports or academic papers. I can't join you tonight, I'm all in. Pro Tip 3/3Physical Context Often used after physical activities to describe the extreme tiredness one feels. After moving furniture all day, we were all in.