noun
A person who interferes in others' affairs, often with unwanted advice or help. Colloquial and mildly derogatory.
彼はいつも人のことに首を突っ込むおせっかい屋だ。
He's a meddler who always sticks his nose into other people's business.
おせっかい屋の隣人がまた余計なアドバイスをしてきた。
The busybody neighbor gave me unsolicited advice again.
おせっかい is the abstract noun or na-adjective meaning 'meddling' or 'nosiness', while おせっかい屋 refers specifically to a person who meddles.
Compound of おせっかい (meddling, from 節介 with honorific お) and 屋 (person with a certain trait). The exact origin of せっかい is uncertain, but it may be related to 節介 (excessive care).