An animal (especially a sheep or goat) that bleats, or a person who complains constantly.
From bleat (Old English blǽtan) plus the agent suffix -er, meaning 'one who bleats.' The suffix -er has been used in English for over 1,000 years to turn verbs into nouns describing the doer of an action.
In old British slang, calling someone a 'bleater' was actually a serious insult—it meant they were a coward or informant to the police. The word's journey from farm animal to human character trait shows how animals' behaviors get projected onto human flaws.
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