can't hack

To be lightweight

Kiana drank two sips of beer and she was wasted. She can't hack.

can't hack it

1) Basically like if you can't handle someone or something.
2) or if you don't understand something

1: "mate did you see the snow outside?!"
"Yeah it's so cold, can't hack it mate!"

2:"Have you started Miss C's work yet?"
"Nah, Its so hard I can't hack it!"