adverb
not altogether; not entirely; not completely
Used in negative contexts to mean 'not completely' or 'not necessarily'. Often followed by ~でもない, ~悪くない, or similar negative expressions to soften a statement.
彼の提案はまんざら悪くないと思う。
I think his proposal isn't altogether bad.
まんざら嘘でも
It's a story that's not entirely a lie.