ויקרא יג:נה פירוש: רש״י, רשב״ם, טור הארוך וSiftei Chakhamim

הֻכַּבֵּס‎ ‎ ‎‎‎‏ אחרי AFTER IT IS WASHED — The verbal form is an expression of something done (a passive infinitive).

פחתת, the word describes a kind of inflammation which eats away at the affected area.

פחתת הוא, “it is an ingrained affliction.” The word is derived from פוחת, something which gradually diminishes, becomes progressively worth less, until eventually it has ceased to have value; in this case the meaning it that the damage will spread until it encompasses the entire fabric.