The word 'yolk' has a straightforward and descriptive origin in Old English, deriving from 'geolca' or 'geoloca', which literally meant 'the yellow part'. This term is directly related to the Old English adjective 'geolu', the ancestor of the modern word 'yellow'. The logic is purely color-based, naming the most visually distinctive component of an egg. Over centuries, the phonetic form simplified from 'geolca' through Middle English 'yelke' or 'yolke' to its current spelling and pronunciation, while its core meaning referring to the yellow, nutrient-rich center of an egg has remained remarkably stable. The secondary, figurative sense of a central or essential part logically extends from this primary biological meaning.

The word 'yolk' is a simplex word of Old English origin. It is not productively formed from smaller, meaningful parts in modern English. The Old English root 'geolca' or 'geoloca' is related to 'geolu', meaning 'yellow'.