。
It goes without saying.
Literal
Not-say-preferably-[classical-ずもがな] is-[copula-だ].
言わずもがな as a stand-alone predicate meaning 'it goes without saying, it's self-evident.' Combines the classical negative 言わず ('not saying') with もがな (the classical particle of wishing/preferring), giving 'would that not be said.' One of those rare classical constructions that has stayed alive in modern Japanese, useful both attributively (something better left unsaid) and predicatively (it goes without saying), with context picking the reading.