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It's when you do a hard cardio workout.
"Man we definitely did a hardio run today with that 10 mile run."
by athena4peace August 12, 2008

Words related to hardio

cardio excercise hard run sweating workout
Intense sexual intercourse consisting of advanced positions, rapid thrusting, accelerated heart rate & a lot of sweat.
After lifting, my girl and I had a serious hardio sesh.
by Meatball1985 July 08, 2014