What does '根筋' in '一根筋' and '少根筋' mean?
筋 means muscle or tendon. The 筋 in these phrases is short for 脑筋, which literally means "brain tendon", but is used to mean "(line of) thought" or "idea".

You can see examples of this word, in phrases like


动脑筋: literally: "to move one's brain tendons", means: "to think hard"
伤脑筋: literally: "to hurt one's brain tendons", means: "a headache" (in the sense of difficult to think about)