"Environmentally-friendly" vs. "Environment-friendly"

I don’t think there’s anything grammatically wrong with environment friendly. It sounds a little funny only because we hear environmentally friendly so much more often, and I think the reason for that is historical.

The environmentally phrases all seem to have taken off at around the same time, during the 1970s, according to Google n-grams. The most common of the phrases at that time was environmentally sound.

Note that environment sound is not an option. It’s clearly ungrammatical, I guess because sound doesn’t take any kind of complement: policies that are sound to the environment is wrong.

The less common phrases apparently followed the lead of environmentally sound, and that’s where we are today.

My guess is that it's not that you're being friendly to the environment. It's that you're being friendly in an environmental way. It's actually odd for me to see the dash in "environmentally-friendly", because the first word in the pair is simply the adverb explaining in what way you are being friendly.

This is probably also informed by the fact that your "environment" can be any number of things from your immediate surroundings up, but "environmental" as an adjective pretty much always refers to something that takes care of "The Environment". We call conservationists "environmentalists", not "environmentists". When you're "environmentally aware", you're aware of how your actions affect The Environment and are aware of ways to minimize your damages. When you're "environment-aware", you simply are aware of your surroundings- that is a term I see passed around my school's computer science labs concerning their robots.

Perhaps it is because environmentally lends itself to other familiar applications; -friendly seems to be one of many. I can think of a few others: environmentally safe, environmentally conscious, environmentally sound, all of which I would consider to be "set phrases", so to speak.

It is worth noting that gay may also be used as an adverb as-is, but you've got me at user- and dolphin-.

I am Australian and Environment-friendly sounds wrong to me, I can't recall ever hearing it in common speech. However a google search revealed several reputable sources using it, including an Australian Government information page.

Wikipedia redirects "Environment Friendly" to "Environmentally Friendly" and points out "eco-friendly" as a synonym.

In my view, there is no easy adverb replacement for gay-friendly, user-friendly or dolphin-friendly, whereas environmentally friendly is easily derived from the adjective environmental.

I offer no evidence to the following, but there may also be a cadence-preference to separate stress in syllables. Consider enVIRonMENTally friendly vs. enVIRonMENT-friendly.