warm the cockles of someone's heart
Unknown, possibly derived either from the similarity in appearance of the heart valves to the cockle, a bivalve mollusk of the family Cardiidae or from a misuse of the medieval Latin word for heart - cochleae cordis. An Irish Gaelic origin has been suggested.
The phrase may be more explained by the nature of the mollusk itself. Its natural environs are usually cold water shoreline areas. The mollusks have to open their shell to ambulate and feed yet, if disturbed or threatened in any way, the shell slams shut to protect its delicate organs.
If the "cockles" are exposed to warmth, especially a cooking temperature, the usually difficult to penetrate shell exposes its entrails and remains open.
The meaning can be interpreted as the "cockles of the heart" are warmed by an emotional experience that exposes the tender and warm side of the human experience, thus opening of the "heart".
- (to provide happiness): warm someone's heart