When Gideon's army was too numerous, the men were taken to a stream to drink, and 300 of them lapped water with their tongue; all the rest supped it up (Judges. vii. 4-7). All carnivorous animals lap water like dogs, all herbivorous animals suck it up like horses. The presumption is that the lappers of water partook of the carnivorous character, and were more fit for military exploits. No doubt those who fell on their knees to drink exposed themselves to danger far more than those who stood on their feet and lapped water from their hands.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894