Tether and Fetter are common because both words refer to a kind of restrain (rope or chain) for an animal.
tether --> tie+together
tet+her....tightly(tet) tie .sth..to her......how?..by rope........
tether- tie+ her
Internet tethering in iphone means.. it can be used as a hotspot to connect or TIE other instruments or equipments to it.