h₂erk-
“to hold, contain, guard”Source of Latin arcēre, arca, English ark, arcane, exercise
Root for holding or enclosing, producing Latin arca (chest) and English ark, arcane, exercise.
Discussion
The PIE root *h₂erk- (to hold, to contain, to guard, to ward off) produced vocabulary spanning containment, defence, and secrecy — all facets of the fundamental act of keeping something enclosed and protected.
Latin arcēre (to keep away, to prevent, to enclose) continues the root and gave English: arcane (arcānus, "shut away, hidden" — knowledge kept enclosed, hence secret), ark (arca, "chest, strongbox" — the container that keeps things safe; Noah's Ark is the enclosing vessel, the Ark of the Covenant is the sacred chest), and arcade (a series of arches — originally an enclosed walkway).
Latin arcus (bow, arch) is from the same root: the arch is a structure that spans and encloses. English arch, archer, arc, and the prefix archi-/arch- in architecture descend from this line.
The word exercise (Latin exercēre, from ex- + arcēre, "to drive out of enclosure") connects back: to exercise was originally to drive livestock out of their pen, then to drill soldiers outside the garrison.
Greek arkéō (ἀρκέω, "to ward off, to suffice, to be enough") shows the semantic development: what wards off danger is sufficient. What holds firm against threat is adequate.
Hittite ḫark- (to hold, to have, to keep) provides Anatolian attestation with the laryngeal preserved as ḫ, confirming the initial *h₂.
The root connects containment (ark, arcade), defence (arcēre), secrecy (arcane), and sufficiency (Greek arkéō) through a single concept: to hold within, to keep enclosed, to maintain a boundary between inside and outside.