arc

From Latin arcēre 'to enclose, shut up, ward off,' with its sibling arca 'chest, box' and the adjective arcānus 'hidden, secret' — literally what is locked away in a box. The same root produced coerce / coercion (co- + arcēre = to fence in, force into compliance) and arcane (arcānus = secret, obscure). The thread running through the family is the idea of walling something in and keeping it shut away.