Mark Ingram Freaks Out Over Derrick Henry's 99-Yard TD Run, 'You Get a Stiffy!'

Derrick Henry had arguably the greatest run in NFL history Thursday night ... and his pal, Mark Ingram, couldn't have been prouder -- 'cause he lost his mind watching it!!!

ICYMI ... Henry tied the longest run ever in the NFL with a 99-yard TD in the 2nd quarter of the Titans' blowout win over the Jags -- and the play was epic.

Derrick stiff-armed three dudes, tight-roped the sideline and outran the entire Jacksonville defense to tie Tony Dorsett's longest-ever TD run.

Ingram -- who won a Heisman Trophy at Alabama just like Henry -- went bananas watching the replay ... super proud of his buddy who's taken criticism for not running well this season.

"You get a stiffy! You get a stiffy! You get a stiffy! I see you, boy!"

Henry's night wasn't over there -- the dude ran for another 139 yards and 3 scores -- and yeah, Ingram was just as pumped for those too.

"Tell them we ain't going for it, bruh! Tell them we ain't going for it! We don't need none of that fake love that they was talking, man! We out here, boy! Yeah!"

Ingram wasn't the only guy impressed ... both Le'Veon Bell AND Ezekiel Elliott praised the gallop on their social media pages.