por no decir

not to say

→ por
phrase Rare #48,510
not to say Used to introduce a stronger or more exact word, often to soften the statement.
SP: Es un desastre, por no decir un fracaso.
EN: It's a disaster, not to say a failure.

Por no decir algo peor.

To say the least.

Sería tonto, por no decir loco, hacer semejante cosa.

It would be foolish, not to say crazy, to do such a thing.