expression, noun
clothes or items bought but never used; unused things taking up space
Idiomatic expression literally meaning 'fertilizer for the chest of drawers'. Used humorously or self-deprecatingly for things, especially clothing, that are bought but never worn or used, just sitting in storage.
このワンピース、買ったけど一度も着てない。完全にタンスの肥やしだ。
I bought this dress but have never worn it. It's completely just taking up space in my closet.
セールで買った服がタンスの肥やしになっている。
The clothes I bought on sale have become unused items in my dresser.
From タンス (chest of drawers) + の + 肥やし (fertilizer). The idea is that unused clothes stored in a dresser are as useless as fertilizer, humorously implying they just sit there and 'rot'.