A lot vs. Alot: 9 Grammatical Pitfalls
c.1400, "an armed force," from Latin armatura "armor, equipment," from armatus, past participle of armare "to arm, furnish with weapons" from arma (see arm (n.2)). Meaning "armor" is mid-15c.; that of "protective covering of a plant or animal" is from 1660s. Electromagnetic sense is from 1835.