c. 1500, "to loll idly, act or rest lazily and indifferently, move indolently if at all," Scottish, a word of uncertain origin. Meaning "recline lazily" is from 1746. Perhaps [Barnhart] it is from French s'allonger (paresseusement) "to lounge about, lie at full length," from Old French alongier "lengthen," from Latin longus "long" (see long (adj.)).
Another etymology traces it through the obsolete noun lungis "slow, lazy person" (c. 1560), which is from French longis "an idle, stupid dreamer," a special application, for some obscure reason, in Old French of the proper name Longis, which is from Latin Longius, Longinus. In old mystery plays and apocryphal gospels, Longinus is the name of the centurion who pierces Christ's side with a spear; the name perhaps was suggested by Greek longe "a lance" in John xix.34. But popular etymology associated the name directly with long (adj.). Related: Lounged; lounging; lounger.