Meaning of “fast” - Learner’s Dictionary

fast

adverb uk us /fɑːst/
QUICKLY

A2 moving or happening quickly:

We ran as fast as we could.
You'll have to act fast.
fast asleep
FIRMLY

in a firm or tight way:

He tried to get away, but she held him fast.
→ See also thick and fast

(Definition of “fast adverb” from the Cambridge Learner’s Dictionary © Cambridge University Press)