What does '根筋' in '一根筋' and '少根筋' mean? The phrase I'm trying to understand is in fact '没那根筋', but I can't find the description for '根筋' in any dictionary.

The search result only came up with '一根筋' and '少根筋'. 

If '一根筋' means one-track mind and '少根筋' means stupid (or perhaps lack of common sense), does '根筋' roughly mean idea/thought?
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)