An idea, inkling, or lead about something; having something come to mind that might explain or relate to a situation.
彼女が怒った理由について心当たりはある？
Do you have any idea why she's angry?
財布を落としたが心当たりがない。
I lost my wallet but have no clue where it might be.
その名前には何か心当たりがあるような気がする。
I feel like I have some recollection of that name.