If something is not said or treated lightly, it is said or treated in a serious way, after great thought.

Accusations like these from a top minister are not made lightly. 高级部长提出这样的指控不可等闲视之。

She is a psychopath, and I don't use the term lightly.

That risks putting people out of work, which you certainly shouldn't do lightly.

Having a child wasn't a decision we took lightly.

I have complete confidence in this guy. And I don't say that lightly.

See also: lightly (NOT SERIOUSLY)