"eternal and unchanging, perpetual, everlasting, having no end," early 15c., from Old French sempiternel "eternal, everlasting" (13c.) or directly from Medieval Latin sempiternalis, from Latin sempiternus "everlasting, perpetual, continual," from semper "always, ever" (see semper-). Compare aeternus from aevum (for which see eternal). Related: Sempiternally; sempiternity. The earlier Middle English adjective was sempitern (late 14c.) from Old French sempiterne and Latin sempiternus.
Trawthe is immortalle, immutable, and sempiternalle.
[Higden's "Polychronicon," 15c. translation]