What is a flat adverb?
When I heard a co-worker tell another co-worker who had just clocked out to "Drive safe!" I snootily thought to myself, That's not grammatically correct! It should be, "Drive safely!"
Are you thinking what I was--that all adverbs must end in -ly?
Adverbs are words that tell us how something is done: safely, quickly, carefully, etc. And they normally end in -ly.
However, it is actually grammatically correct to tell someone to "Drive safe!" (And it's also just downright nice.) That's because "safe" is a flat adverb. An adverb that doesn't end in -ly is called a flat adverb.
Of course, you could also wish that someone would "Drive safely," but why use a normal adverb when you could use a flat adverb?