押忍 (hiragana おっす, romaji ossu)
A contraction of おはようございます (ohayō gozaimasu, “good morning”, also said when first meeting a commonly-met acquaintance for the first time that day). First used in the Budo Senmon Gakko, later among other martial artists.[1]
The kanji spelling is an example of ateji (当て字).
