The phrase to warm the cockles of one’s heart means to give one a comforting feeling of contentment.
The noun cockle now denotes specifically an edible burrowing bivalve mollusc with a strong ribbed shell common on sandy coasts (Genus Cardium, family Cardiidae). But it was formerly applied more vaguely to other bivalves and their shells.
This is because this word is from Old French coquille, meaning shell (with the English shifting of the stress, the original Middle-English forms such as cokille have become cockle, in the same way that gentle is from Middle-English forms such as gentile). In his textbook Lesclarcissement de la langue francoyse (1530), the teacher and scholar of languages John Palsgrave translated both the English terms coccle fysshe and cokell shell as French coquille.
The expression the cockles of one’s heart is first recorded in Some Observations upon the Answer to an Enquiry into the Grounds and Occasions of the Contempt of the Clergy (London, 1671), by the English satirist John Eachard (1636?-1697):
I am confident of it, that this Contrivance of his did inwardly as much rejoyce the Cockles of his heart, as he phansies, that what I writ did sometimes much tickle my spleen.
PROBABLE ORIGIN OF THE COCKLES OF ONE’S HEART
The phrase might be based on the resemblance between the shape of the heart and that of a cockleshell – or of the body the shell protects.
In Tractatus de Corde (A Treatise on the Heart – 1669), the English physician and physiologist Richard Lower (1631-91) wrote, about the muscular fibres of the ventricles:
Fibræ […] spirali suo ambitu Helicem sive cochleam¹ satis aptè referunt.
These fibres […], with their spiral circuit, may rather aptly be referred to as a Helix or snail-shell¹.
(¹ In Traité du Cœur, the 1679 French translation of Lower’s treatise, cochlea is rendered as coquille de limaçon, snail-shell.)
In classical Latin, cochlea, or coclea, meant snail and snail-shell – cf. English cochlea, denoting the spiral cavity of the inner ear, from the resemblance of its shape to that of a snail-shell.
The ventricles of the heart might therefore have been called cochleæ cordis², and this might have been turned into the cockles of one’s heart.
(² The word cochleæ is the nominative plural of cochlea; cordis, meaning of the heart, is the genitive singular of cor, heart—cf. English courage.)
According to a popular theory, the explanation lies in the zoological name for the cockle, Cardium, from Greek καρδία (= kardía), heart (cf. English cardiac).
But this theory overlooks the fact that the cockles of one’s heart is first recorded in 1671, before the Swedish natural historian and physician Carolus Linnaeus (Carol von Linné – 1707-78) founded modern systematic botany and zoology in Systema Naturæ. The earliest instance of Cardium in the sense of cockle that I have found in an English text is from Lectures on the Materia Medica (London, 1773), by William Cullen, professor of medicine at the University of Edinburgh:
These were formerly confounded partly with the Fishes; for the reasons of classing them separately, vide Linnæus, vol. I. Systema Naturæ. […] Cardium edule, the Cockle; Cardium echinatum, prickly Cockle.
According to another (and far-fetched) theory, cockle in the expression is from the Latin term of endearment corculum, little heart, diminutive of cor. For example, in Folk-etymology, a dictionary of verbal corruptions or words perverted in form or meaning, by false derivation or mistaken analogy (London, 1882), A. Smythe Palmer wrote:
In default of a better I make the following suggestion. As we find ‘corke,’ a provincial word for the core or heart of fruit, so ‘cockle’ may be for ‘corcle,’ ‘corkle,’ or ‘corcule,’ an adaptation of the Latin ‘corculum,’ a little heart, and the expression would mean the core (French ‘cœur’), or “heart of heart,” but why the word occurs in the plural I cannot say.
But the word corcle, from Latin corculum, is first recorded in the early 19th century, long after the cockles of one’s heart.