The phrase "butt kicker" means someone or something that motivates or pushes you to work harder, overcome challenges, or achieve your goals. They push you to do your best and don't let you give up easily. Full definition
Whether you have a little girl or boy, this black snapsuit is sure to turn your little tyke into a little butt kicker for the whole family to enjoy. (abckidsinc.com)
Followed with butt kickers, with heels kicking back to touch your butt, for 20 seconds. (womenshealthandfitness.com.au)
10 Seconds - Double Butt Kickers — Essentially you will be running in place but be sure to bring your feet back and as far up to your buttocks as possible. (consumerhealthdigest.com)