Here are the sentences including the word.
では おまえたちがここへくるまでのいきさつをいちおう記録するから正直にのべるんだ。
まあ こんなところでよかろう まったくよくもくだらん まねばかりしおって
I'm not sure if it is a colloquial form of 「しまう」.
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Sign up to join this communityIt's not related to しまう. おる in this context is an auxiliary verb which is described in this definition:
3 動詞の連用形に付いて用いる。
㋐(相手を軽蔑する気持ちを込めて)…やがる。「あんなやつに負け―・って」
Basically it's a word to show one's disgust. I think there's no direct equivalent auxiliary verb in English, but you can use anything that can make a sentence dirty and rough; "hell", "shit", etc.
The Osaka-ben equivalent is よる (eg まねばかりしよって), which may be more commonly heard in the daily life of an ordinary person.