From World Wide Words.

It’s one of the more lovely idioms in the language, isn’t it? Something that warms the **** of one’s heart induces a glow of pleasure, sympathy, affection, or some such similar emotion. What gets warmed is the innermost part of one’s being. It’s not that surprising that it should be associated with the heart, that being the presumed seat of the emotions for most people. But what are the ****?

We’re not sure. We do know that the expression turns up first in the middle of the seventeenth century, and that the earliest form of the idiom was rejoice the **** of one’s heart.

**** are a type of bivalve mollusc, once a staple part of the diet for many British people (you may recall that Sweet Molly Malone once wheeled her wheelbarrow through Dublin’s fair city, crying “**** and mussels, alive, alive oh!”). They are frequently heart-shaped (their formal zoological genus was at one time Cardium, of the heart), with ribbed shells.

It may be that the shape and spiral ribbing of the ventricles of the heart reminded surgeons of the two valves of the ****. But I can’t find an example of the word **** being applied to the heart outside this expression, which makes me suspicious of this explanation. It may be that the shape of the ****, suggesting the heart as it so obviously does, gave rise to **** of the heart as an expansion.

After this piece appeared in the Newsletter, James Woodfield pointed out that there is another possible explanation. In medieval Latin, the ventricles of the heart were at times called cochleae cordis, where the second word is an inflected form of cor, heart. Those unversed in Latin could have misinterpreted cochleae as ****, or it might have started out as a university in-joke. Oddly, cochlea in Latin is the word for a snail (from the shape of the ventricles—it’s also the name given to the spiral cavity of the inner ear), so if this story is right we should really be speaking of warming the snails of one’s heart.