'yolk-filled' is a Modern English compound formed by combining the noun 'yolk' and the past-participle/adjectival form '-filled' (from the verb 'fill'). 'yolk' itself comes from Old English 'geolca'. 'fill' comes from Old English 'fyllan'.

'yolk' developed from Old English 'geolca' to Middle English 'yolke' and then to modern English 'yolk'; 'fill' developed from Old English 'fyllan' to Middle English 'fillen' and then to modern English 'fill'. The compound 'yolk-filled' is a productive modern formation using the adjective-forming pattern noun + -filled.

Initially 'yolk' specifically named the yellow part of an egg; over time the word retained that meaning. The compound 'yolk-filled' straightforwardly means 'filled with yolk' in modern usage.