BT Ket 77b(21)var. :ושאדי אר̇{{ו}}^י^̇שיה עד דרפייא"דרטיב" ארעיתא דמוחיה‏ : he pours it on his head until the bottom of his skull becomes soft/moist/
See the entire entry for rṭb vb. to be moist

BT Ket 77b(21) :ונטיל ליה תלת מאה כאסי ושאדי אר̇{{ו}}^י^̇שיה עד דרפייא ארעיתא דמוחיה וקרע ל^י^ה למוחיה‏ : he takes 300 cups and pours it on his head until the base of his skull softens and he cuts open his skull
See the entire entry for rpy vb. to be lax