noun which may take the genitive case particle 'no', noun
staunch; hardcore; dyed-in-the-wool
Describes a person who is deeply committed, unwavering, or thoroughly imbued with a certain quality or ideology. Often used with nouns like 保守派 (conservative), ファン (fan), etc.
彼は筋金入りの保守派だ。
He is a dyed-in-the-wool conservative.
筋金
As a hardcore baseball fan, I watch every game without fail.
noun
metal reinforcement
Original literal meaning: having metal wires or bars embedded for strength. Now mainly used in historical or technical contexts, such as describing reinforced concrete or traditional construction.
この古い建物の壁には筋金入りの補強が施されている。
The walls of this old building are reinforced with metal.