In Old English, the word petersilie was the only term used to refer to parsley, but then when the Normans invaded England, they brought with them the Old French word, peresil. Those forms briefly coexisted for a while, shifting in spelling over time to the point where they were utterly confused and our current word parsley emerged from the ashes. Both the Old French and Old English nouns trace to Latin petroselinium, which is an oddly appropriate compound of the words petros, meaning "rock", and selinon, meaning "celery". Linguists are unsure about the origins of both of those roots, but they probably come from a Pre-Greek language. According to Google NGrams, usage of the word parsley took off in the 1960s, peaked in the 1980s, and has been on the decline since then.
Adam Aleksic, a rising sophomore studying linguistics and government at Harvard University, has been described as the internet's sixth most famous etymologist. He also has disturbing interests in politics, vexillology, geography, board games, conlanging, and law.