adverb
completely; absolutely; totally
Used with a negative sentence to mean 'not at all' or 'by no means'. Emphasizes the complete absence or impossibility of something.
彼の言うことは万に一つも信じられない。
I can't believe a single word he says.
そんなことは万に一つもありえない。
That is absolutely impossible.
全然 is a more common and versatile adverb for 'not at all', while 万 is more emphatic and often used in fixed expressions like 万に一つも.
絶対に means 'absolutely' and can be used in both positive and negative sentences, whereas 万 is restricted to negative contexts.
Derived from the kanji 万 (ten thousand), used here in an extended sense to mean 'by any chance' or 'in any way', which evolved into an emphatic negative adverb.