some time ago; the other day; once
Refers vaguely to a past occasion, often when the speaker cannot recall the exact time. Can also be used in a rhetorical or polite way to refer to a previous meeting.
Thank you very much for your help the other day.
いつぞやそんな話を聞いた気がする。
I feel like I heard that story some time ago.
Derived from いつ (when) + ぞ (emphatic particle) + や (question particle), literally 'when was it?'. The expression fossilized into an adverb meaning 'at some indefinite past time'.