Meaning

noun perennial northern temperate plant with toothed leaves and heads of small purplish-white flowers

adjective (of time) constituting the full extent or duration

originFrom Middle English live long, leve-long, lefe long (as in Alle the lefe longe daye), equivalent to lief + long. Compare Dutch heel den lieven langen dag (“all the livelong day”), German der liebe lange Tag (“the livelong day”) and German die liebe lange Nacht (“the livelong night”).

Etymology from Wiktionary, CC BY-SA 3.0