Refers to worthless or shoddy items, often with a dismissive tone. Can be used for physical objects or abstract things like ideas.
この店の商品は駄物ばかりだ。
The goods in this shop are all trash.
彼の提案は駄物だった。
His proposal was low-grade stuff.
Compound of 駄 (da, 'poor quality') and 物 (mono, 'thing'). The exact historical derivation is uncertain, but the spelling is conventionally associated with the meaning.