Small saltwater clams or shellfish; in the phrase 'warm the cockles of one's heart', it means to make someone feel happy or loved.
From Middle English, ultimately from Greek 'konche' meaning 'shell'. The phrase 'cockles of the heart' dates to the 1600s, possibly because cockles are heart-shaped.
We have no idea why cockles are connected to the heart in that phrase—it might be because the shells are vaguely heart-shaped, or it could be a completely lost medieval metaphor that just stuck around.
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