informal: a potato
The noun spud is related to Old Norse spjōt, meaning spear, German Spieß, of same meaning, and English spit in the sense of skewer. It is first recorded in in the English-Latin dictionary Promptorium parvulorum sive clericorum (Storehouse for Children or Clerics - around 1440):
Spudde, cultellus vilis (= a cheap small knife).
This word was later used to denote a pronged instrument for digging or weeding. For example, the Irish satirist, poet and Anglican cleric Jonathan Swift (1667-1745) wrote, in A Pastoral Dialogue (circa 1729):
My love to Sheelah is more firmly fixt,
Than strongest Weeds that grow those Stones betwixt:
My Spud these Nettles from the Stones can part;
No Knife so keen to weed thee from my Heart.
From this noun, the verb to spud means to dig up (something) by means of a spud. Over the course of time, the name for the digging implement used to dig up potatoes was applied to the latter. An early example is found in a letter written on 30th January 1850 and published in The Waterford Evening News (Ireland) of 2nd February (Milesian means Irish):
MY THOUGHTS ABOUT LAND AND LANDLORDS
You direct my attention to the ‘Milesian’s’ letter lately published in The News. I regret that a clever man like the ‘Milesian’ should fall in with the landlord cry, and cheer on those who, from time to time, have enslaved the bold Milesian race and broken their hearts. [...] It now, I think, comes with a bad grace from a Milesian to tap once more the conqueror on the back.
He is for Protection! So would I too if I saw the country prosperous under it. [...] If I saw the Irish farmer, who is ever toiling, living like a farmer in other countries, eating good bread, and meat, and butter, and eggs, and cheese, taking his tea and coffee every day—eating the ‘spuds’ or ‘Murphys’ only once in place of three times a day, then I would be for Protection.
As there is another man of my name with whose opinions I sometimes disagree, and wishing that no one should charge me with the ugly crime of personation, I beg to subscribe myself your obedient servant, not “Charles,” but
The noun spud in the sense of potato came to be used figuratively to denote “any very diminutive person or thing”, and the adjective spuddy was used to mean “stumpy”, according to John Greaves Nall in Dialect and Provincialisms of East Anglia (1866).
The word spuddy was also a noun. In London Labour and the London Poor (1851), the English journalist and social researcher Henry Mayhew (1812-87) wrote:
Of the Nicknames of Costermongers
Like many rude, and almost all wandering communities, the costermongers, like the cabmen and pickpockets, are hardly ever known by their real names; even the honest men among them are distinguished by some strange appellation. [...] “Spuddy” (a seller of bad potatoes, until beaten by the Irish for his bad wares).
The sense development of the noun spud, from the digging instrument to that which has been dug up with this instrument, is comparable to that of Latin pastinaca, the ultimate origin of English parsnip. This Latin noun, denoting the carrot and the parsnip, was derived from the verb pastinare, meaning to dig and trench the ground in order to prepare it. This verb is in turn from the noun pastinum, denoting a kind of two-pronged dibble, hence the digging and trenching of the ground.