How does “rest easily” differ from “rest easy”?

Question

The New York Times (August 17) reported that the N.F.L. has decided to cracked down on several players who wear fierce and funky face masks to make them look tougher or cooler under the headline, “Uniform police give face masks a closer look.” The N.F.L. considers the use of such accessorized helmets without a legitimate medical reason violates the league’s uniform policy. http://www.nytimes.com/2013/08/18/sports/football/NFL-uniform-police-give-face-masks-a-closer-look.html?ref=sports?src=dayp&_r=0

The article reads:

“Gone is Brian Orakpo’s corrugated headgear, modeled after the Batman villain Bane, and Darnell Dockett’s square-bottomed mask, which looked oddly like a panini grill. Tuck’s face mask fans, however, can rest easily. He provided N.F.L. officials with documentation, signed by the team doctor and head athletic trainer, certifying that his mask protected his neck by preventing opponents from grabbing it.”

I was drawn to the phrase, “Tuck’s face mask fans can rest easily.” I can find the idiom, “rest easy” in the meanings of “(1) not to worry - CED” and “(2) go to sleep without worries – OED” in any of English dictionaries at hand, but no dictionary carries “rest easily” as an idiom, because I think, it’s a simple combination of verb and adverb.

However, I’m interested in what’s the difference is between “rest easy’ shown in “This insurance policy will let you rest easy “ shown as a sample in OED and “rest easily” in “Tuck’s face mask fans can rest easily” in the above quote.

Does the addition of two letters - ‘ly’- make any difference? If it does, what’s the difference?

Answer 1

Dictionaries sometimes give a specific definition for rest easy (to relax and stop feeling worried) because both it and the virtually identical breathe easy aren't exactly being used literally. You probably weren't really having difficult breathing before something relaxed you and allowed you to "breathe easy".

But they won't be likely to define rest/breathe easily because that's normally only used in an absolutely literal sense. If you have trouble with your sinuses, you might take a decongestant to help you breathe easily.

One can thus understand easy in such contexts as meaning at your ease, in a relaxed manner (cf take it easy), as opposed to the more literal easily = without undue difficulty.

That distinction isn't absolute though. OP's citation is an instance of rest easily being used figuratively, and people also sometimes use rest/breathe easy in the literal sense.