Meaning/Usage: Very tired
Explanation: "Beat" can mean to lose. It can also mean to be hit over and over again. When someone loses or they are hit over and over again, they are either mentally tired or physically tired. This phrase is saying you are very tired for whatever reason.
"I had a tough day today. I'm totally beat."
"I don't want to go out tonight. I'm beat."
"I'm beat so I'm going to sleep early today."
A: "Hey Steve, we're going to go to a club tonight. Wanna come?"
B: "I worked a 12 hour shift today. I'm beat so I'm going to just stay home tonight."
Other Common Sentences
"I'm very tired."
"I'm exhausted."