noun
influential quarters; authoritative sources
Used in news and formal contexts to refer to people or groups with influence who provide information, often anonymously.
有力筋によると、来月にも新制度が発表される見通しだ。
According to influential sources, the new system is expected to be announced as early as next month.
Compound of 有力 (influential) and 筋 (line, channel, source).