It derives from the war games that inspired D&D.
The 1971 chain mail The miniature games that inspired D&D included a one-on-one table that decided on hits or failures based on the attacker's weapon and defender's armor. This appears to be the origin of D&D's term "armor class", which refers to the chain mail Regulate. The meaning seems to be similar to "classification", how to sort, order or group different things.
The term "class" is widely used all over the world chain mail Rules related to a category, group, rank, classification, or type of thing, including weapons, armor, cannons, heroes, wizards, elementals, and the undead. "Class" is used in a similar way in war games Don't give up the shipwhere it is used to categorize different ships based on tonnage. This may have been a common naming convention for wargamers in the 1970s.
The man-to-man fight rules chain mail Provide a table to determine the throw that needs to be hit. The weapon type is taken into account in comparison to the armor type. Some armor types share a column in the melee table. e.g. Leather and padded armor are functionally the same. The ranged weapon table refers to the "class of armor worn by the defender", with a number assigned to each of the eight armor groups.
The original from 1974 Dungeons offers an alternative combat system that comes very close to that chain mail Table, with the exception that the number is now specifically referred to as "armor class" and the higher level instead of the weapon type determines the number that must be rolled to get a hit.
Originally "armor class" simply meant "type of armor worn", with the result that some armor is better than others. Since most monsters do not wear armor and there are other magical means to increase the armor class in D&D, the term "armor class" was soon decoupled from the actual armor types and only referred to the number of hits. However, it has largely been retained from D&D tradition, which has always preserved many archaic words and ideas as part of their taste.