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(not) at all; completely; utterly; entirely; altogether
very; extremely
彼の言い訳はてんで話にならない。
His excuses are not worth discussing at all.
彼はてんで役に立たなかったみたいだ。
It seems he was of no use at all.
彼女の説明はてんでわからなかった。
I couldn't understand her explanation at all.