The reason the word "beat" is used, is to suggest one is as tired as one who has been beaten up by someone.

'Beat', when used as an adjective, means "worn-out; fatigued." Therefore, "I'm beat." simply means: "I'm fatigued/exhausted/very tired." Another colloquialism would be to say: "I'm spent."

Beat means bad shape, it is a slang For example: I have a beat up car ( my car is in bad shape, it looks ugly) I'm so beat means I am in bad shape

Hello Vivian, In a certain context it could also mean "lacking anything". " I am beat for my cash." " I am beat to my socks." ( lacking everything) Synonyms (when it means worn out or exhausted) : " clapped out", " on one's last legs" , "wiped out", "zonked" .