Something that cheeps; a young bird or small animal that makes cheeping sounds.
From 'cheep' (the sound) + '-er' agent suffix. The agent suffix creates nouns for things that perform an action—in this case, things that make cheeping sounds.
A 'cheeper' is adorably informal—you might hear someone call chicks or baby birds 'little cheepers'—and it shows how English speakers instinctively create agent nouns for animals based on the sounds they make!
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