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.
(..) This is something NASA expressly rejected. We're talking about mutiny here, which is not a word that I take lightly. So we do this together or not at all. And before you answer, consider the consequences. If we mess up the supply (..)2015 The Martian