It's grammar.
It comes from the Latin grammatica.
Whoever spells it with an "e" is ****ing retarded.
Grammar. Just look it up in the dictionary.
Seriously, there's no other correct way to spell it. I might accidentally type grammer, but that's just a typo.
I did look it up, grammar seems right, but i'm still not gonna dismiss this idea, look at the word colour/color, I spell it colour, yet some/most? Americans spell it color. Same goes for Metre/Meter, if you fell the same way to them, as you do to "Grammar" then this comes down to where you live, and how you were taught to spell it.