A person who butts in or intrudes into situations and conversations where they are not wanted; a meddler or busybody.
Early 20th-century American slang from butt in + -sky/-ski suffix. The -ski ending was added for humorous effect, potentially referencing Eastern European ethnic backgrounds in a stereotypical way.
Buttinsky became famous through cartoons and casual American speech—it's a beautiful example of how slang invents new words by gluing together verbs and fake suffixes, and the result feels completely natural even though it's totally made up.
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