used to show that what you are talking about is much more extreme, etc. than your words suggest :
The result was unfortunate, to put it mildly (= it was extremely unfortunate) .
used to show that what you are talking about is much more extreme, etc. than your words suggest :
The result was unfortunate, to put it mildly (= it was extremely unfortunate) .
Oxford Advanced Learner's English Dictionary. Оксфордский английский словарь для изучающик язык на продвинутом уровне. 2005