What is the origin of the phrase plumb forgot?

The phrase "plumb forgot" likely originated from the use of the Word "plumb" as an adverb meaning "completely" or "absolutely." In this context, "plumb" emphasizes the degree to which something is forgotten. The Word "plumb" in this sense dates back to the 16th century, while the specific phrase "plumb forgot" likely emerged later as a colloquial expression to convey total forgetfulness.