“Tether” is the older word, but with either “rope” or “tether” the saying goes back many centuries.
In original usage the expression “to come to the end of one’s rope” alluded apparently to a cow or other domestic animal, perchance a dog, staked for grazing or for protection by a rope attached to its neck.
The range of the animal was thus limited. If a cow, it could not graze beyond the limit of its rope; if a dog, it could not get at a stranger, no matter o v fiercely might run or lung; who kept outside the circle of its tether.
Thus, “to come to the end of one’s rope” had the literal meaning, to reach the limit of one’s resources.
But both rope and tether early became sinister synonyms for the hangman’s rope.
From this, the saying acquired a second and more devastating meaning by which we use it to signify that one has been effectually checked in the commission of crime, has reached, figuratively, if not literally, the noose of the hangman.
It is probable that this extended meaning was effected, in part at least, by the old proverb, “Give him enough rope and he’ll hang himself.”