least vs to the lowest degree

least is a noun but to the lowest degree is not a noun.

Word	Noun	Adjective	Verb	Adverb
least	Yes	Yes	No	Yes
to the lowest degree	No	No	No	Yes

As adverbs, least and to the lowest degree are synonyms defined as:

least and to the lowest degree: used to form the superlative, smallest in size or degree

least (noun)	to the lowest degree (noun)
something that is of no importance

least (adjective)	to the lowest degree (adjective)
the superlative of `little' that can be used with mass nouns and is usually preceded by `the'; a quantifier meaning smallest in amount or extent or degree

least (adverb)	to the lowest degree (adverb)
used to form the superlative, smallest in size or degree	used to form the superlative, smallest in size or degree