・What could you possibly have been thinking of [about] when you did it? あのとき, 一体君は何のことを考えていたのでしょうか.

2 〔…のことを〕よく考える, 思案する, 熟慮する (deliberate, meditate) 〔about, of, over〕.

・Think before you speak. よく考えてから口をききなさい.

・I'll think about it. まあよく考えておきましょう《しばしば体(_てい)のいい謝絶》.

・when [if] you think about it よくよく考えてみると.

・He thought about quitting his job. 仕事を辞めようかと考えた.

・I have a hundred and one things to think about [of]. 思案しなければならないことが山ほどある.