expression, i-adjective
completely unknown; having no clue about; unexpected
Used when one has absolutely no idea or recollection about something; often in negative contexts like 心当たりのない請求 (a bill one has no memory of).
See also: 心当たりがない
そんなメールは心当たりのないものだった。
That email was something I had no clue about.
心当たりのない請求書が届いた。
I received a bill that I have no memory of.