"to descend, dismount," Old English alihtan, originally "to lighten, take off, take away," from a- "down, aside" (see a- (1)) + lihtan "get off, make light" (see light (v.)). The notion is of getting down off a horse or vehicle, thus lightening it. Of aircraft (originally balloons) from 1786. Related: Alighted; alighting.
"on fire," early 15c., apparently from Middle English aliht, past participle of alihton (Old English on-lihtan) "to light up," also "to shine upon" (see light (n.)).