Origin and history of spud

mid-15c., spudde, "small, stout knife or dagger of poor quality" (a sense now obsolete), a word of uncertain origin, probably related to Danish spyd, Old Norse spjot "spear," German Spiess "spear, lance."

The meaning "small-bladed spade for cutting" is from 1660s; the sense of "short and thick person or thing" is from 1680s; that of "potato" is recorded by 1840 in an English traveler's account of American peculiarities. Other extended senses in 19c. American English include "a baby's hand," "a piece of dough baked in fat."

Spud, a short scurvy Knife, a Short-arse, or little despicable Fellow. [Phillips' dictionary, 1706]

Spud barber (1935) was prison slang for "potato-peeler."