Plural of cockleshell; the shells of cockles (small edible clams), or light, flimsy boats.
From cockle (a type of shellfish from Old English 'coccel' and Latin 'conchylia') plus shell. Also used metaphorically for fragile boats, as cockle shells are delicate and thin-walled.
The term 'cockleshells' became famous from the nursery rhyme 'Rub-a-dub-dub, three men in a tub'—but historically, it was also a derisive term for the small British gunboats sent to dangerous waters, showing how language turned humble shells into symbols of brave but seemingly doomed vessels.
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