Cockles of the heart is a noun phrase meaning “the core of one’s being” often used in the phrase “warm the cockles of the heart”.
The idiom derives from the Latin term “cochleae cordis”, the verticals of the heart.
The saying “warm the cockles of the heart” means to warm and gratify one’s deepest feelings.
The expression was first recorded in Some Observations upon the Answer to an Enquiry into the Grounds and Occasions of the Contempt of the Clergy by the English satirist John Eachard in 1671.
The term had huge popularity in the first two decades of the 19th century and then after a short pause from 1850 until 1959 when its use started to decrease.
However, “cockles of the heart” is still used, even on social media sites such as Reddit or Twitter, and the phrase “warms the cockles of my heart” was added to Urban Dictionary in 2017.