The go, you mean? The definition I saw said it was a "traditional Japanese unit of liquid volume," so I suppose it could be obsolete. Oh, wait! Found this on Wikipedia:
"Japan: one cup is 1/5 litre (200 mL). A traditional Japanese cup (g�?) is 180 mL; 10 Japanese cups together make one shou, the traditional flask size, 1.8 litres. Sake is typically sold by both the cup (180 mL) and flask (1.8 litre) sizes. The cup size used for measuring rice is the traditional size of 180 mL. (For example, a 10-cup rice cooker has a capacity of 1.8 litre or 1 shou.)"
So it may have earned the name because it's one-fifth of a liter, yes?
Hmm, those are all very awkward numbers, clearly this unit isn't part of any western measurement standards. I just wonder if it's equivalent to five of a smaller denomination in the same Japanese system. Oh well, I assume it's obsolete now?
Uselessness, I found this: One go is 180.39 milliliters, 0.3812 U.S. pint, or 0.3174 British imperial pint. Don't know whether that helps at all. The only other reference to the Japanese word refers to a board game and doesn't seem to have much to do with measurements.