Single Mindedly, Intently, Earnestly, A Line
noun
You have one muscle, therefore it only does one thing. That's it. It is single mindedly doing that one thing. It is intently and earnestly doing that one thing.
ひとすじ
The reading is a bit weird. It's the kun'yomi readings, where 一 is ひと and 筋 is すじ. Look back on the vocab words that use these readings to remember this one.
うちの長男は野球一筋なのよ。女っ気も無くてさ。
Our oldest son does nothing but play baseball. He doesn’t have a girlfriend either.