10:50 AM PT -- Brady's message is getting Fiddy's stamp of approval ... 'cause the rap star just responded to the shout-out, saying, "Oh sh*t he gonna do it again, he listening to (Many Men) getting his mind right. @tombrady let’s go!!!"

"Many men wish death upon me" -- 50 Cent.

They said he was too old, they said he lost a step ... and now Tom Brady is having the last laugh ... channeling his inner rap star to clap back at the haters after a big playoff win!!

Brady posted a video Monday on Instagram bragging about the 41-28 win over the Chargers -- and punctuated his message with 50's "Many Men."

Specifically, Brady used the hook with the following lyrics directed at his critics:

"Many men wish death upon me. Blood in my eye, dog, and I can't see. I'm trying to be what I'm destined to be, and n****s trying to take my life away."

Yeah, Brady's going gangsta.

Obviously, Brady heard some of the chatter before Sunday's game -- where football pundits were saying he's fallen off and the Pats were screwed.

In his video, Brady said ... "Monday, January 14, it's great to be working on a Monday. Great team win. We're on to Kansas City, baby!! Let's go. LET'S GO!"

Clearly, TB12's got the memory of an elephant ... and is putting all reporters on notice.