h₂erḱ-eyo-
“to keep warding off, defending”ark, arcade, arcane
Iterative of *h₂erḱ- giving Latin arca/arcēre, English ark, arcade, arcane.
Discussion
The PIE form *h₂erḱ-eyo- (to keep warding off, defending) is an iterative derivative of *h₂erḱ- (to hold, to guard, to ward off), with the *-eyo- suffix expressing ongoing or repeated action. The meaning is "to be in the continuous act of protecting" — defence as a sustained posture, not a single gesture.
Latin arcēre (to keep away, to prevent, to enclose) is the primary reflex, with the iterative sense preserved in the legal and architectural vocabulary: arcane (arcānus, "shut away, hidden" — that which is kept enclosed, hence secret), ark (arca, "chest, strongbox" — a container that keeps things enclosed; the Ark of the Covenant is the sacred chest, Noah's Ark is the enclosing vessel), and arcade (arcāta, a series of arches — originally an enclosed walkway, a covered passage that wards off weather).
The architectural sense deserves emphasis. Latin arcus (bow, arch) is from the same root: the arch is a structure that spans and encloses, that wards off the weight above it. From arcus English derives arch, archer, arc, and the combining form archi-/arch- in its architectural sense (distinct from the Greek arkhi- "chief" which is from a different root). The Roman aqueduct's arches, the Gothic cathedral's pointed arches, and the modern bridge's steel arches all continue the PIE image of a protective span.
Greek árkein (ἀρκεῖν, "to ward off, to suffice, to be enough") shows a semantic development from "protecting" to "being sufficient" — what wards off danger is enough, what is adequate protects. The derivative arktos (bear) is sometimes connected: the bear as "the one who wards off/protects," though this etymology is debated.
The Hittite form ḫark- (to hold, to have, to keep) provides crucial Anatolian attestation, with the laryngeal *h₂ preserved as Hittite ḫ. The Hittite evidence confirms the initial laryngeal and the basic "hold/keep" meaning as the root sense, from which "ward off" and "enclose" developed.
The word exercise (Latin exercēre, "to keep busy, to train" — from ex- + arcēre, literally "to drive out of enclosure") shows how even the fitness sense connects back to the root: to exercise was originally to drive livestock out of their enclosed pen, then to put soldiers through their paces outside the garrison. The modern sense of physical training preserves the Roman military metaphor.