From 握(にぎ)り (nigiri, “連(れん)用(よう)形(けい) (ren'yōkei, “stem or continuative form”) of the verb 握(にぎ)る (nigiru).”) + 拳(こぶし) (kobushi).
握(にぎ)り拳(こぶし) • (nigiri-kobushi)