I came across the expression from the following dialogue.
After a boy told an old man with this sentence.
しっかりしねえか おっちゃん! いったい なにがあったんだ きょう!
The old man punched him at the face and said this sentence.
やかましい つべこべぬかすな てめえなんざ けぇっちまえーっ
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Sign up to join this communityけぇっちまえーっ means 帰{かえ}っちまえ. けえる is a working-class accent of かえる in Tokyo. It is 東京下町ことば. A sound of "ai" changes to that of "ee" in that accent in some words such as ちげーねー(違いない), でぇーこん(だいこん). http://www.muse.dti.ne.jp/~squat/tokyoben.htm
「~ ちまえ」is a rough way of saying 「~ てしまえ」. So けぇっちまえーっ is something like "Just get out!".