commonplace and stale; humdrum and hackneyed
Describes ideas, expressions, or things that are both ordinary and trite, lacking freshness or originality. Often used in critical or literary contexts.
His speech was commonplace and stale, and no one was moved.
The movie's plot was humdrum and hackneyed, and I quickly got bored.
平凡 means 'commonplace' or 'ordinary' without the strong connotation of staleness; 平凡陳腐 adds the nuance of being trite and overused.
陳腐 means 'stale' or 'hackneyed'; 平凡陳腐 combines this with 'commonplace' to emphasize both lack of originality and ordinariness.
A compound of 平凡 (commonplace) and 陳腐 (stale, hackneyed). The exact historical derivation is uncertain, but it is a straightforward combination of two Sino-Japanese words.