OR for them its just random words Ex-Wing, Bee-Wing, Shklee-Wing.... and later WE adapted our alphabet on what those ships looked like and their weird names.
New ships in Rogue One
The special effects crew would nickname the models with X-Wing Y-Wing etc and the nick names made their way to the general public.
Not all the nicknames made it though. The Millennium Falcon's nickname of "Flying Hamburger" didn't make it.
I would love to see the Assault Frigates on screen,the Bulk Cruisers and CC-9600s as well (The CC-7700 and CC-2200 Interdictors appeared in the Tarkin novel and the 9600 was a very good capital sized assault ship so it would fit IMO.)