also: しゅすじ
noun
close associates of the boss; inner circle
Refers to the people who are close to and trusted by the leader or head of a group, often in a somewhat old-fashioned or yakuza-related context. Not a common everyday word.
組長の主筋にあたる者たちが集まった。
The people who are close to the boss gathered.
側近 is a more common and neutral term for a close aide or confidant, while 主筋 has a stronger nuance of being part of the leader's inner circle in a hierarchical group.
Compound of 主 (しゅ, 'master, boss') and 筋 (すじ, 'line, connection'). The exact historical derivation is uncertain, but it likely originated in underworld or feudal contexts to denote those directly connected to the leader.