To be on one

American Slang

Definition

Acting unusually intense, hyped, or erratic—often “doing the most,” sometimes due to strong emotions or being under the influence.

Examples

"She came into the meeting on one, talking a mile a minute and steamrolling everyone."

"Dude was on one at the party—jumping on tables, yelling, just doing the most."

"He's on one after those energy drinks, firing off texts and picking fights over nothing."