means Containing, resembling, or diluted by water — and by extension, weak, thin, or lacking in strength or flavor.

from A straightforward Old English build: 'wæter' (water) plus the adjective-forming '-ig,' the same suffix that turns 'thirst' into 'thirsty' and 'sleep' into 'sleepy.' Water itself traces back to a Proto-Germanic root '*watar' and beyond it to a Proto-Indo-European '*wod-,' a deep ancestor shared by the Greek 'hydor' and the Russian 'voda' (yes, the same 'voda' lurking inside vodka). So the word is as old as English gets, and its drift toward meaning 'weak' or 'thin' simply follows what water does to anything you pour it into.