He is a dyed-in-the-wool conservative.
筋金入りの野球ファンとして、毎試合欠かさず見ている。
As a hardcore baseball fan, I watch every game without fail.
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.
この古い扉は筋金入りで、とても頑丈だ。
This old door is reinforced with metal and very sturdy.