n. & v.
n. 1 a rope etc. by which an animal is tied to confine it to the spot. 2 the extent of one's knowledge, authority, etc.; scope, limit.
v.tr. tie (an animal) with a tether. at the end of one's tether having reached the limit of one's patience, resources, abilities, etc.
[ ME f. ON tjóthr f. Gmc ]