In the PHB, some weapons have a length that begins with "c" and some others don't.
What does "c" mean in this context?
As far as I can tell, the use of c. in the Weapon Types, General Data, and "To Hit" Adjustments table isn't defined anywhere in the Player's Handbook (1978).
However, I've always taken that c. to mean circa. While usually employed in conjunction with estimating historical dates, using circa to mean “approximately” isn't unheard of, and Gygax seems to be doing that here.
For example, this would mean that the c. 4' entry for the battle axe's length means that a battle axe is about 4 feet long.