the.com/fallback

means A backup option or plan you switch to when the first choice fails or becomes unavailable.

from Built plainly from 'fall' plus 'back' — the physical sense of falling or stepping backward to a previous, safer position, like troops retreating to a defensible line. Military and strategic writing used 'fall back' this way for centuries before the noun 'fallback' peeled off to describe the position itself — the thing you retreat to rather than the act of retreating.