The act of using the palm of one's hand to slap another person's butt, in an effort to encourage them to speed up or keep doing a job well done. Originated in American Football circles and hence is usually male to male, but can be done in a variety of combinations.
Can also be used as a congratulatory mannerism after a key play in a given sport. Usually from Coach to Player.
Little Billy had just completed a 50 yard run for a touchdown in the finals and as he came off the field his coach issued him a hustle slap to show his approval.