KC Chiefs rookie QB Patrick Mahomes has a rocket for an arm -- in fact, he once threw a baseball 96 MPH!!! 

Mahomes was a baseball star in high school before focusing on football -- plus his father, Pat Mahomes, was an MLB pitcher for 12 years.

So, we asked if he could still fire the rock -- and he told us he really believes he could hit 100+ if he got back in the baseball groove.

For the record, Patrick once threw a football 62 mph -- on video.

If he says he can hit 100 mph with a baseball ... we kinda believe him.

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