noun
utter failure; completely wrong guess; big miss
Used when a prediction, expectation, or attempt is completely off the mark. Often carries a colloquial, emphatic tone.
天気予報が大外れで、ピクニックは中止になった。
The weather forecast was a complete miss, so the picnic was canceled.
彼の予想は大外れだった。
His prediction was way off.
外れ means a miss or failure, but 大外れ intensifies it to a complete or spectacular failure.
Compound of 大 (おお, 'big') and 外れ (はずれ, 'miss, failure'). The exact historical derivation is straightforward; the word is a transparent emphatic form.