expression, noun-modifying (irregular)
trustworthy; reliable; credible
Fixed prenominal expression meaning 'worthy of trust'. Used to describe people, sources, or information that can be relied upon.
He is a trustworthy person.
I heard it from a reliable source.
Compound of 信頼 (trust) + に (particle) + 足る (to be sufficient/worthy). The verb 足る is a classical form of 足りる, giving the expression a formal, literary tone.