Nathan is shouting K’ihap which in many Korean martial arts, like Taekwondo is a form of Kiai, a martial arts battle cry.
Kiai appears to be a compound of ki (気) meaning mind, breath or spirit and ai(合, 合い), a stem of the verb awasu (合わす), “to unite”; therefore literally “concentrated or united spirit”. However, Frederick John Lovret notes, “One should note that ai, the conjunctive stem of the verb au, does not mean “to join” in this case: when used in the second position of a compound word, ai becomes an emphatic marker. Kiai, therefore, should be translated as “spirit!”, not “spirit-joined”.
