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.
(..) But you have some strength in you,my dear Hobbit. What happened,Gandalf? -Why didn't you meet us? -I am sorry,Frodo. I was delayed. A friendship with Saruman is not lightly thrown aside. One ill turn deserves another. It is over. Embrace the power of the Ring... ...or embrace your own destruction! (..)2001 The Lord of the Rings: The Fellowship of the Ring
(..) Goodbye. I suppose you think that was terribly clever. Come on,Gandalf. Did you see their faces? There are many magic rings in this worId,and none of them should be used lightly. It was just a bit of fun. Oh,you're probably right,as usual. -You wiII keep an eye on Frodo,won't you? -Two eyes. (..)2001 The Lord of the Rings: The Fellowship of the Ring