The adverb has 4 meanings:
Meaning #1:
with low volume
Synonym: softly
Meaning #2:
with little or no sound
Antonym: noisily (meaning #1)
Derived from adjective: quiet (meaning #2)
Meaning #3:
with little or no activity or especially agitation; (`quiet' is a nonstandard variant for `quietly' as in"sit here as quiet as you can")
Synonym: quiet
Antonym: unquietly (meaning #1)
Derived from adjective: quiet (meaning #1)
Meaning #4:
in a restful manner
Synonym: restfully
