- Alternative form of full well
In ordinary modern English, adverbs modify adverbs, so the phrase fully well would be standard. However, the phrase full well, a surviving usage of full itself as an adverb, is more common than fully well, which is sometimes considered incorrect or a hypercorrection. This is most often used to modify the verb to know.
- Brians, Paul. Common Errors in English Usage. Williams, James Co. 2009.