Age quod agis is a well-known saying which is a sort of Latin call to mindfulness: do what you are doing. Be here now in the activity at hand.
Legend has it that Johnny Ringo was an educated man. (Not so: a story for later.) But so he is depicted over and over. In this scene from Tombstone, the best of the movies about Doc Holliday and the shoot-out at the O. K. Corral, Ringo trades Latinisms with the gun-totin' dentist, who was indeed an educated man and a fearless and deadly gunslinger to boot, his fearlessness a function of his 'consumption.'
I don't mean his consumption of spirits, but his tuberculosis. His was the courage of an embittered man, close to death.
The translations in the video clip leave something to be desired. Age quod agis gets translated as 'do what you do best'; the literal meaning, however, is do what you are doing. Age is in the imperative mood; quod is 'what'; agis is the second person singular present tense of agere and means: 'you do' or 'you are doing.'
Curiously, Doc Holliday did not die with his boots on. He died in bed.