to say the least

Adverb

idiomatic not-comparable

(idiomatic) Used to suggest an understatement.

Japanese Meaning

控えめに言えば、実際の程度を過小評価して表現するための言い回し / 表現を穏やかに、または控えめに示すための慣用句

After the storm damaged the roof and flooded the basement, the situation was dire, to say the least.

嵐で屋根が破損し、地下室が浸水した後、その状況は控えめに言っても深刻でした。

Sense（1）

(idiomatic) Used to suggest an understatement.

The 9/11 attacks weren't very good for America. / To say the least!