source: Are you stupid? ［Fate/Stay Night 2014］ : animegifs

at a loss

1. puzzled or uncertain what to think, say, or do:

〖be ～〗 «…するのに/…に» 困って, とまどって «to do/for» ; «…か» 途方にくれて «(as to)wh節･句»

e.g. with infinitive : she became popular, and was at a loss to know why

e.g. he was at a loss for words.: 彼はなんと言ってよいかわからなかった。

2. making less money than is spent buying, operating, or producing something:

〖文尾で〗赤字で, 損失を出して〈経営する･販売するなど〉

e.g. a railroad running at a loss.