"Shunt" comes from the use of the resistor as a shunt, which means that it is connected across something. When mechanical/analog meters were used, they usually deflected full scale at some small voltage (commonly 75 mV). So you would choose a shunt resistor that would have 75 mV of drop at the required full-scale defection of your meter, and connect it directly across the meter input terminals, creating a shunt across the meter terminals, for current to flow through. They are still used this way, but usually the meter is digital. And "shunt" resistors (as opposed to generic power resistors used as such) are often still specified with 75 mV full-scale, so e.g. 75 mV @ 100 A. This is awkward for digital meters, which would be better served with a decimal scaling like 100 mV @ 100 A.