This is a problem that should not be 'taken lightly'. [=this is a problem that should be treated seriously]
Considering the harm he did, he's 'gotten off lightly' with only a warning.
Our troops were armed too 'lightly' to take on the heavily armed invaders.
(..) Something the High Sparrow told me. You've spoken with him? I promised him I wouldn't tell anyone. If he found out I told... It would be a breach of confidence which he would not take lightly given his constant prattling about the truth. I am your mother. You can always trust me. (..)2016 TvShow: Game of Thrones Title: Book of the Stranger Season: 6 Episode 4