noun
dimwit; blockhead; person who is useless despite being present
Derogatory idiom literally meaning 'a lantern at noon'—someone who is as useless as a lantern in broad daylight, often implying inattentiveness or lack of common sense.
あいつは昼行灯で、何を頼んでもまともにできない。
That guy is a dimwit; no matter what you ask him to do, he can't do it properly.
会議中ずっとぼんやりしていて、まるで昼行灯だ。
He was spacing out the whole meeting—he's like a lantern at noon.