The full version of this expression is “warm the cockles of your heart”, although it is often shortened to just “warm the cockles”. So, where does it come from? There are several theories about this.
The first and by far the most popular theory I have found takes into account the shape of cockles, which are a kind of mollusc, and which are roughly heart-shaped. Cockles were a popular food source, and this shape would have been well known in earlier times, and could have given rise to the expression.
In a similar vein, another proposal is that “cockles” was closely linked with the French word coquille, which means “shell”, and it became associated with the human heart, as some claim that the heart’s chambers are shell-shaped.
The next theory I shall look at suggests that the expression may derive from Latin. The heart ventricles are sometimes known in Latin as cochleae cordis. This is from cochlea, meaning “snail shell” or “spiral”, from the shape, while cordis is from cor, which means heart. It is not difficult to see how “cockles” could have developed from an anglicised pronunciation of cochleae.
What none of these theories take into account so far is the “warm” part of the expression. And this, of course, brings me onto the next theory. The word “cockle” is not only used to mean “mollusc”, but can also refer to a chamber in a kiln. The heart, of course, has four chambers, and this could mean that our idiom is a metaphor likening the heart to a kiln, obviously something very warm once the chambers have been fired, just as the experience “fires” or “warms” your heart – or its cockles!
Finally, another alternative explantion for this idiom is that the word “cockles” here derives from the German word Kachels. This word refers to the tiles that would generally have been used to cover fireplaces and traditional ovens, or kachelovens. The idea here is that “warm the cockles of the heart” is a corruption of “warm the Kachels of the hearth”, obviously something desirable to keep the home warm and cook the meals!
Personally, I find the last theory the least plausible, as I have not found a lot of evidence to suggest that people generally referred to their fireplaces as Kachels. Instead, I like to think that perhaps more than one theory may have part of the answer. There is no reason why we should not assume that Latin cochleae helped us to associate “cockles” with the heart, while a pun on the kiln “cockle” served to develop the idiom, for example!