Spud

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late Middle English (denoting a short knife): of unknown origin. The sense ‘potato’ (dating from the mid 19th century) was originally slang and dialect.

wiktionary

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From Middle English spudde(“small knife”). Origin unknown; probably related to Danish spyd, Old Norse spjót(“spear”), German Spieß(“spear; spike; skewer”). Compare English spit(“sharp, pointed rod”). The use of the term for a potato was originally in British dialect and slang usage.

etymonline

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spud (n.)

mid-15c., "small or poor knife," of uncertain origin probably related to Danish spyd, Old Norse spjot "spear," German Spiess "spear, lance"). Meaning "spade" is from 1660s; sense of "short or stumpy person or thing" is from 1680s; that of "potato" is first recorded 1845 in New Zealand English.