'Threshold' is likely related to 'thresh' (to trample) — the plank worn smooth by countless feet.
The strip of wood or stone forming the bottom of a doorway; by extension, the point of beginning or entering, a limit below or above which something changes.
From Old English 'þrescold' or 'þerscold,' from Proto-Germanic *þreskuldaz. The first element is likely related to Old English 'þrescan' (to thresh, to trample, to stamp), from PIE *terh₁- (to rub, to turn). The second element '-old' is of uncertain origin — possibly related to Proto-Germanic *wulduz (a plank) or simply an obscure suffix. The literal sense would be 'the thing you tread upon' or 'the stamped-upon plank' — the worn piece of timber at the bottom of a doorway, polished by countless footsteps. Key
Medieval folk etymology connected 'threshold' to the practice of laying thresh (straw) on the floor to absorb moisture and dirt — the 'thresh-hold' being the board at the door that held the thresh inside. While linguists reject this derivation, it reflects the real domestic function of the doorsill as a barrier keeping floor coverings from spilling out.