noun
imitation that looks identical to the original; fake causing the original to lose value
A yojijukugo (four-character idiom) used to describe a counterfeit so convincing that it devalues the genuine article. Rare and literary; not used in everyday conversation.
魚目燕石のごとき贋作が市場に出回り、本物の価値が下がった。
Counterfeits that are like 魚目燕石 (fish eyes and swallow stones) flooded the market, lowering the value of the genuine articles.
The exact derivation is uncertain; the spelling is conventionally associated with the meaning of a deceptive imitation. The characters 魚 (fish) and 目 (eye) suggest a fish eye that resembles a pearl, while 燕 (swallow) and 石 (stone) suggest a stone that looks like a swallow's egg, both symbolizing convincing fakes.