noun
children's game of trading members via rock-paper-scissors
A traditional Japanese children's game where two lines sing a call-and-response song, then play janken (rock-paper-scissors) to win members from the opposing team. The name is often associated with the song's lyrics.
See also: じゃん拳 (じゃんけん)
子供たちが公園で花いちもんめをして遊んでいた。
The children were playing 'hana ichimonme' in the park.
じゃんけん is the rock-paper-scissors game itself, while 花いちもんめ is a specific group game that incorporates janken as a deciding mechanism.
The exact origin is uncertain. The name likely comes from the song's lyrics, with '花' (flower) and '一匁' (one monme, an old unit of weight) possibly referring to a flower-selling chant or a play on words.