はっきよい

• はっきょい (hakkyoi)

Various theories exist:

• The Japan Sumo Association referee ruleset states it is a shift from a compound of 発気 (hakki, literally “discharging spirits”) +‎ 揚々 (yōyō, “exultant, triumphant”).^[1]
• According to Masatsugu Okazaki, a shift from 早競へ (haya-kioe), a compound of 早 (haya, “already, now”) +‎ 競へ (kioe, the 已然形 (izenkei, “realis form”) and/or 命令形 (meireikei, “imperative form”) of classical verb 競ふ (kiou), “to compete”).
• The pronunciation might have originally been はっけよい (hakkeyoi), see that article for more.

はっきよい • (hakkiyoi) 

1. (sumo) alternative spelling of はっけよい (hakkeyoi): said by a 行司 (gyōji, “referee”) after a successful 立ち合い (tachiai, “initial charge”) and when both wrestlers are in a standoff