I’m not an etymologist but …
The surface used to get seaplanes in and out of the water actually was called a “sea plane ramp” since when they were leading edge aviation tech. The few survivors are still called "seaplane ramp"s today. And the ramp part refered to the fact it’s an inclined plane between two levels, shore & water.
As sea planes gave way to land planes I’d bet ramp got appropriated for the concrete surface in front of the terminal. Remembering this was all happening in an era where nothing was paved unless it had to be; grass was the default surface for everything.
As to apron …
Waay back in the day, airports were simply 1/2 mile or 3/4 mile squares of flat grass-covered former farm field with a small terminal building in one corner. Planes landed & took off on the grass, and parked in front of the building to load & unload.
Given the reality of rain & snow, the area in front of the building would get muddier faster thatn the rest of the field. Between plane traffic, fuel truck traffic, cargo wagon traffic & passengers (& horses), churning the surface to mud would be pretty much a sure thing.
A pretty obvious innovation would be to pave a more-or-less fan-shaped area in front of the building where the airplanes could park.
Which when viewed from above, would look like an apron; a protective semi-circular wrap around the working side of a person. Sorta like the way the dirt area of a traditional baseball diamond looks with the terminal building being at home plate.
My pet peeve in aviation terminology is “tarmac” for the aircraft parking area of an airport. Somehow the word finds its way into every news article about airline ops.
“Tarmac” is simply a synonym for “asphalt”. Back in 1930 or whenever, it would have made sense to refer to the paved area as such. “The mighty DC-3 pulled up onto the asphalt; soon the crowd would greet the celebrities”, etc. “Asphalt” in this sentence meaning the paved parking area in the otherwise grass airport.
But in 2011? Please newspeople, give “tarmac” the decent burial it deserves.
In the business in the US, we call it the ramp. I don’t know of anyone who uses another term. Some Canadian airports call it the apron.