"In English grammar, a flat adverb or bare adverb is an adverb that has the same form as a related adjective.
It does not end in "-ly", e.g. "drive slow", "drive fast". A flat adverb is sometimes also called simple adverb."
from Wikipedia: Flat adverb
Flat adverbs enter the contemporary usage of English in two ways: On the one hand, they are residues from "long time ago" before in the 18th
century scholars deemed them wrong and insisted on the suffix "-ly" to make an adjective an adverb.
On the other hand, the suffix - that is often perceived as bulky - is simply dropped in informal language and slang.
- I can see you crystal clear (Adele: Rolling in the deep)
- Drive slow, slave, friendly! (street signs)
- Think different! (Apple campain)
- it works fine
- I exercise daily
- "Live fast, die young" (fast is a "true" flat adverb, young an adjective)
- I can't wait that long
- a low-paid worker
- Live loud! (EMP slogan)
- I only bought these saucepans because they were going cheap
- They were selling these items off cheap (one could argue, though, that "cheap" is here indeed an adjective describing the items)
- Let us see how loud you scream
- wounds cut deep (Paradise Lost: Our saviors)
- to stoop low (a fixed phrase featuring a "true" flat adverb)
- it is freezing cold (Cambridge dictionary categorizes freezing
both as an adjective and an adverb)
- delving deep
Many optional flat adverbs have survived in often used phrases or fixed expressions:
- play it safe
- take it slow
- Words don't come easy (Song by F.R. David)
- sleep tight
- hold tight
- to go easy on somebody
- it works great (note that "it works greatly" would not be a viable alternative)