penitus

Penitus is an adverb built on the same root as penetro and penus, the inner store of a house. It works in two directions at once: physically "deep inside", and figuratively "right through, utterly". With verbs of destroying, changing or knowing it is doing the second job and is best read as an intensifier - penitus delere is to wipe out entirely, not to destroy at a depth.

Two things to keep apart. There is a rare adjective penitus, penita, penitum, "inward", which does exist but is not what you are usually looking at. And penitus is not related to paene, "almost", though in unpunctuated manuscript hands the two can be misread for one another, and their senses pull in opposite directions. English gets penetrate, penetralia and, distantly, penates from the family.