Etymology of Baa
The baa of a sheep. The first written references to this sound are found in 1580-1590. It is extremely likely that it existed before that period.
In German: bäen. A sheep’s cry is described as a baa, baaing or bleating.
The word baa has been used in literature and magazines for a long time. Examples include The American Lawyer (“I suppose we are all familiar with the old story of the school reader, where the lawyer advised his client to play crazy, and got a sheep’s baa for his fee.”), John Steinbeck’s Holiday (“In this autumn season it is ineffably still — not a bird’s cry, not a sheep’s baa, not the chirr of a grasshopper, not a rustle of wind in grass.”) and Vanity Fair (“He baas like a billy goat in tin-can heaven.”); all literature uses baa extensively.
Durkin, Philip. The Oxford Guide to Etymology. Oxford: Oxford University Press, 2009.
Wedgwood, Hensleigh, and J. C. Atkinson. A Dictionary of English Etymology. 2d ed. London: Trübner &, 1872.