The weak, thin cry made by sheep or goats; or to make such a sound; can also mean to complain or whine in a thin, annoying way.
From Old English 'blætan' meaning to bleat or cry out, likely imitative in origin. The word has been used since at least the 9th century to describe the characteristic sound of goats and sheep, and later developed the secondary meaning of complaining.
The evolution of 'bleat' from an animal sound to human complaining shows how language absorbs onomatopoeia—words that imitate sounds—and then extends them metaphorically; we hear the thin, persistent quality of a bleating sheep and apply it to annoying human whining.
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