noun
A physical prop or brace used to support something, such as a pole or stick placed against a wall or door to keep it from moving.
倒れそうな塀につっかいを当てた。
I put a prop against the fence that was about to fall over.
ドアが閉まらないように、つっかいをかませて
I wedged a brace to keep the door from closing.
つっかえ棒 is a more specific term for a prop stick or bar, while つっかい is a broader noun for any prop or support.
Derived from the verb つっかう (to prop up), with the continuative form used as a noun. The exact historical derivation is uncertain.