The word "少根筋" is formed by directly combining the character 少 (meaning "to lack"), 根 (meaning "root", but here used idiomatically as a counter for tendons), and 筋 (meaning "tendon" or, figuratively, mental acuity), resulting in a literal interpretation of "lacking a tendon", which serves as a metaphor for describing a person who is scatterbrained or deficient in common sense.