I'm curious why is ゴミ commonly spelled in Katakana. What word is it derived from, if any?
Let me attempt to give an English explanation of @macraf 's description for you.
This word gomi was used mainly by farmers and meant 木の葉 ("[fallen] tree leaves"), and gomi is still used in various dialects to refer to [fallen] tree leaves.
In Nagano Prefecture in Japan, people in some places use the word gomi to refer to tree leaves, and for fallen pine needles, they use the term matsu gomi.
In the Aichi prefecture dialect, fallen pine needles are just called ゴ. In Kyoto, there is also the expression 「ゴを掻く」 [literally "to scratch leaves", idiomatically meaning "to rake leaves"].
Later on, ゴミ was used to refer to things that were considered worthless, 取るに足らないもの or 役に立たないもの, in other words, things that were useless.
(I would appreciate any comments or fixes to this attempt at translation.)