This letter is modified from the Hebrew (aleph = an ox), which was meant to indicate the outline of an ox's head.
Among the Egyptians, a is denoted by the hieroglyphic which represents the ibis. Among the Greeks it was the symbol of a bad augury in the sacrifices.
In logic, A is the symbol of a universal affirmative. A asserts, E denies. Thus, syllogisms in bArbArA contain three universal affirmative propositions.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894