not know the time of day 一问三不知

“一问三不知”，指怎么问都不知道（say "I don't know" to every question），很糊涂或很无知。可以翻译为“be entirely ignorant; not know a thing”。与英文惯用语“not know the time of day”意思相近，表示“to be completely unintelligent, daft, or not alert”。

例句：

别问他了，他总是一问三不知。

Don't ask him any questions. He never knows the time of day.