To baa is to make the noise a sheep makes; as a noun, a baa is that noise. Small children learn it as the farm-animal sound, the way they learn moo for a cow. Adults use it too, usually with a smile, or when they are talking to a child. The word does not hide a smaller word inside it: people copied the noise. German has close cousins of that copy, bäh and mäh. Though actually, English has not stood still. Speakers once said this word with a flatter vowel, closer to the sheep. That vowel later moved back in the mouth — a slow change in English in the 1700s and 1800s — so ‘baa’ on the page is a slightly faded copy of the noise. Something similar happened when older ‘mew’ for a cat gave way to ‘meow’. Informal, slightly childlike in tone, but not only a children’s word.