Use completely to describe something that is totally and utterly...something. A completely ridiculous idea is absolutely crazy and stupid.
Completely is an adverb that comes from the Latin completus, "to fill up." We use it to mean "entirely" or "wholly." So if a building is completely destroyed, no part of it is left standing. Reading a newspaper story or watching a documentary film completely means you finished it from beginning to end. And doing so can completely change your view on an issue.