clenched fist
Refers to a tightly closed hand, often in the context of determination, anger, or physical readiness.
He stood up, clenching his fist.
Compound of 力 (ちから, 'power') and 拳 (こぶし, 'fist'). The exact historical derivation is uncertain, but the combination naturally conveys a 'power fist' or 'clenched fist'.