The word 'potato' has one of the most unusual etymologies in English: it is a hybrid of two words from two completely unrelated language families spoken on two different continents, merged by Spanish colonizers who conflated two distinct New World plants.
The story begins in the Caribbean. When Columbus and subsequent Spanish explorers arrived in Hispaniola and the surrounding islands, they encountered a starchy root vegetable cultivated by the Taino people. The Taino word for this plant was 'batata,' and the plant was what we now call the sweet potato (Ipomoea batatas, a member of the morning glory family). The Spanish adopted the word as 'batata' and brought both the plant and the word back to Europe.
Decades later, Spanish conquistadors in the Andes encountered a completely different tuber: the potato (Solanum tuberosum, a member of the nightshade family). The Quechua-speaking peoples of the Inca empire called it 'papa' — a word that is still used in most of South America. The two plants are botanically unrelated (they belong to different families), look different, taste different, and grow in different climates. But to the Spanish, they were both starchy underground tubers from the New World, and the names were blended: the Taino 'batata' and the Quechua 'papa' merged into Spanish 'patata.'
English borrowed the Spanish form in the 1560s as 'potato.' For the first century of its English existence, 'potato' referred primarily to the sweet potato, since that was the plant Europeans knew first. The true potato arrived in Europe slightly later (probably in the 1570s, brought to Spain from Peru) and was initially called the 'Virginia potato' or 'bastard potato' to distinguish it from the sweet variety. Over time, the qualifier was dropped, and 'potato' came to mean the Andean tuber exclusively, while the Caribbean plant was relegated to 'sweet potato.'
Other European languages took different etymological paths. German 'Kartoffel' derives from Italian 'tartufolo' (little truffle), because early Europeans compared the underground tuber to a truffle — both were mysterious edible objects dug from the earth. Russian 'картофель' (kartofel) was borrowed from German. French 'pomme de terre' means literally 'apple of the earth' — a calque that parallels Dutch 'aardappel' (earth apple). Italian uses 'patata' (from Spanish). Only Quechua-influenced South American Spanish retains 'papa' for the ordinary potato.
The potato's impact on world history is almost impossible to overstate. After overcoming initial European resistance (the potato was feared as a member of the deadly nightshade family and was associated with leprosy and other diseases), it became the most important food crop in northern Europe. The potato enabled population booms in Ireland, Germany, Poland, and Russia. The Irish Potato Famine of 1845–1852, caused by potato blight, killed approximately one million people and drove another million to emigrate, fundamentally reshaping the demographics of Ireland, Britain, and the United States.
The English slang 'spud' for potato has a separate etymology: it originally meant a short knife or digging tool (possibly from Scandinavian 'spjot,' a spear), and transferred to the potato because potatoes are dug from the ground with such tools.