From ground +‎ nut. Compare Icelandic jarðhneta, German Erdnuss (both cognate with earthnut) and Hungarian földimogyoró.

groundnut (plural groundnuts)

1. A climbing vine, Apios americana, of eastern North America, having fragrant brownish flowers and small edible tubers.
2. Any similar plant having underground tubers.
3. The nutlike tuber of such a plant, especially peanuts.
4. A plant which bears its nutlike seeds underground, such as a peanut (Arachis hypogaea) or a Bambara groundnut (Vigna subterranea).

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  Groundnuts in their skins; Arachis hypogaea; the peanut in the shell
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  Groundnut in the skins; Arachis hypogaea; the peanut in the shell
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  Groundnut pulse with membrane; Arachis hypogaea; the peanut nut with its skin on
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  Groundnut skin; Arachis hypogaea; the peanut shell
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  Tamil groundnut snack; a ball of peanut halves
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  Roasted groundnuts in their membranes; roasted peanuts in their skin
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  Groundnuts; Arachis hypogaea; Peanuts
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  Groundnut seller; peanut vendor
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  Groundnut oil cake; peanut oil cake

• earthnut
• tree nut

• “groundnut”, in OneLook Dictionary Search.
• groundnut on Wikipedia.Wikipedia