expression, godan verb (-ru)
flex one's biceps; make a muscle
Idiomatic expression literally meaning 'make a strength lump'; used when someone flexes their arm to show off their biceps.
彼は力こぶを作って見せた。
He flexed his biceps and showed them off.
子供が「見て見て」と言いながら力こぶを作
The child made a muscle while saying 'Look, look!'