dog's age

Noun

US colloquial

(US, colloquial) A long time; many years.

Japanese Meaning

非常に長い時間、もしくは何年にも及ぶ長い期間を表す表現

I haven't seen him in a dog's age, so I almost didn't recognize him.

彼には長い間会っていなかったので、ほとんど彼だとわからなかった。

Sense（1）

(US, colloquial) A long time; many years.

John Higgins, I haven't seen you in a dog's age.