Apex entered English from Latin, where it originally meant "a small rod at the top of a flamen's cap." What's a flamen's cap? Flamens were priests who devoted themselves to serving just one of the many ancient Roman gods (for instance, just Jupiter or Mars). Those priests wore distinctive conical caps that English speakers dubbed "flamen's caps." Sixteenth- and seventeenth-century dramatist Ben Jonson was one of the few English writers known to have used "apex" in its flamen's-cap sense: "Upon his head a hat of delicate wool, whose top ended in a cone, and was thence called apex."