A somewhat literary or formal word for words expressing resentment, complaint, or reproach. Not an everyday term for a simple complaint.
His words of resentment were the accumulation of years of discontent.
The letter was filled with bitter complaints and reproaches.
Compound of 怨 (resentment, grudge) and 語 (word, language). The exact historical derivation is uncertain, but the meaning follows directly from the kanji components.