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1.
no problem, it's cool.

Usually used in response to someone's apology to indicate acceptance.

See No worries.
Hey,

Sorry I insulted you earlier. I didn't mean for it to come out that way.

Response: no harm no foul buddy.
by Ryan February 22, 2005
 
2.
I think this phrase actually began in basketball. If there isn't any harm inflicted to stop a game, then there should be no foul called.

It's used commonly today to brush off insults or mistakes and the like.
Gerchewd: Hey sorry - my butt just accidentally grazed your arm.
Doughlas:Hey man, no harm no foul.

Gerchewd: Yyyyyeah.
by Fistatron November 03, 2009
 
3.
If a phrase that originated on the basketball court. It is in reference to referees or players on the streetball court calling soft fouls.

In the eyes of some, soft fouls being called only slow the game and detract from the contest and beleive that unless the contact is significant there is no point in calling the foul. Hence, No harm, no foul.
Example A
Player A: 'you fouled me'
Player B: 'come on man, no harm no foul'

Example B
Referee blows whistle for soft hands foul
Player: 'come on man, no harm no foul'
by thirdhalf January 13, 2010