expression, godan verb (-u)
to put it mildly; to understate; to speak with restraint
Used when deliberately softening a statement, often to avoid sounding extreme or rude. Frequently appears as 控えめに言っても (even if I put it mildly) to introduce a strong opinion.
彼の態度は、控えめに言っても失礼だった。
His attitude was, to put it mildly, rude.
その計画は控えめ
That plan is, to say the least, reckless.