I actually did not encounter the word until I came to Edinburgh. The word Onomastics comes from the Latin word Onomasticus (ie. Onomasticon Goidelticum, is the place name book for Ireland) and before that Greek based on the ending (Greek neuters end in -on) so I assumed that the word ment placename in particular not just name (cf. Latin "nomen"; see also here). My friend actually does river names in particular but he says that he does onomastics.
In any case, you could be completely correct since I am basing my analysis solely on usage.
