Dictionary of American Regional English (Belknap Press, 1985), p.403:
2. A drinking fountain, esp. one outdoors; orig. the valve which caused the water to bubble in such a fountain.
esp. freq. in WI
[Earliest recorded example dated 1911]
It looks like the tag has been added already, but for the record I just wanted to supply what the DARE had to say about it (which I had been meaning to do since...well, the beginning of February). I thought it was just WI but DARE has found it in a wider swath of land than I expected.
Nth refers to n (northern) IA, n ID, n IL, n IN, MI, MN, MT, ND, New England, n NJ, NY, n OH, OR, n PA, n SD, WA, WI, n WY
NMidl means North Midland and refers to northern DE, central and south IA, c IL, n MD, NE, s NJ, c OH, central and south PA, s SD, n WV