perpetuous (adj.)

mid-14c., perpetuel, "everlasting, unceasing, existing indefinitely, continuing forever in future time;" late 14c., "uninterrupted, continuous," from Old French perpetuel "without end" (12c.) and directly from Latin perpetualis "universal," in Medieval Latin "permanent," from perpetuus "continuous, universal," from perpetis, genitive of Old Latin perpes "lasting," probably from per "through" (from PIE root *per- (1) "forward," hence "through") + root of petere "to seek, go to, aim at" (from PIE root *pet- "to rush, to fly").

Related: Perpetually. Perpetual motion in reference to a hypothetical machine which, being set once in motion, will continue forever unless stopped by some external force" is attested from 1590s.