expression, ichidan verb
make a firm resolution; resolve firmly
Used when someone solidifies their determination or makes up their mind after deliberation.
彼は留学する決心を固めた。
He made a firm resolution to study abroad.
なかなか決心を固められない。
I just can't make up my mind firmly.
Compound of 決心 (determination) + を + 固める (to harden/solidify).