LX-Logical Xoice (choice-lame, I know)
I think trim "names" came about in the 50's or something when car trims came in Luxury (LX), Deluxe (DX), and Exectutive (EX). My dad mentioned this to me when I got my Civic DX, and we got a crack out of it because my Civic isn't all that "Deluxe". I think they just evolved into abbreviations over the years. There's some trims you can't make into words any more, like GXE.