expression, noun
none of your business; mind your own business; keep out of it
A set phrase used to reject unwanted meddling or advice, often with irritation. The お世話 is ironic, implying the help is not appreciated.
「もっと勉強したほうがいいよ」「大きなお世話だ！」
"You should study more." "Mind your own business!"
彼のアドバイスは大きなお世話だった。
His advice was completely unwelcome.
お世話 normally means help or care, but in 大きなお世話 it is used ironically to mean unwanted interference.
A fixed expression combining 大きな (big) and お世話 (help/care), where お世話 is used ironically to mean meddling or unwanted concern.