Etymology
1. Borrowed from Late Old Swedish pærun (“pear”) (compare dialectal Finnish Swedish pärun, perun and standard Swedish päron). Originally meant “pear”, in which sense it has been attested since the time of Mikael Agricola (output c. 1543–1552); meaning shifted to “potato” in the late 18th century (first attested in 1787). Doublet of päärynä.
2. Details on semantic shift
3. When potatoes were first introduced to Finland in the 18th century, they were usually called maaperuna (literally “earth-pear”) (compare Swedish jordpäron (“potato”), German Erdbirne (“potato”, literally “earth-pear”), Grundbirne (literally “ground-pear”)). “ground/earth apple” is also found in neighboring languages, such as German Erdapfel, French pomme de terre, Russian земляно́е я́блоко (zemljanóje jábloko), and may be older than “ground/earth pear”, which might originate from German.
4. A similar name 'earth-pear' or 'earth-apple' is used in some languages of another root vegetable, the Jerusalem artichoke: English ground pear, Dutch aardpeer, Norman paithe dé tèrre, Russian земляна́я гру́ша (zemljanája grúša), Armenian գետնախնձոր (getnaxnjor), Turkish yer elması.
5. peruna in its modern sense is first mentioned in a 1787 dictionary by Christfried Ganander. As the potato grew in relevance, there was pressure to shorten the name to simply peruna, as potatoes quickly become much more relevant for Finnish speakers than pears. For the sense “pear”, the doublet päärynä came about as a more recent borrowing from Swedish, and ambiguity was thus avoided.
6. An occasional folk etymology states that this term comes from the name of the country of Peru, because that is where the modern cultivated potato originates from. See: Swedish Nouns '
päron'.