noun
valued customer; profitable customer
Business term for a customer who brings significant profit or value to a company. Often used in sales and marketing contexts.
この店の常連はみんな為筋だ。
All the regulars at this shop are valued customers.
営業部は為筋を大切にしている。
The sales department cherishes its profitable customers.
Compound of 為 (tame, 'benefit' or 'sake') and 筋 (suji, 'line' or 'connection'), literally meaning a connection that brings benefit. The exact historical derivation is uncertain, but it is used in modern business Japanese.