also: かびがはえる
expression, ichidan verb
get moldy; get mouldy
Literal meaning: something becomes covered with mold or mildew.
パンにカビが生えた。
The bread got moldy.
expression, ichidan verb
become outdated; get out-of-fashion
Idiomatic use: something (often an idea, trend, or person) becomes stale, old-fashioned, or past its prime.
そのギャグはもうカビが生えてるよ。
That joke is already moldy (outdated).