A back-formation from Middle English pease, whose final s looked like a plural ending.
Try pea in the lookupFills the word field on the homepage. Nothing is sent until you submit.
The sequence below summarizes the cited forms, dates and evidence. It is not a complete scholarly history.
Pea is supported by Merriam-Webster, AHD, and the Online Etymology Dictionary as a back-formation from Middle English pease, which was reinterpreted as a plural. Earlier forms include Old English pise, from Latin pisa/pisum, from Greek pison.
This page summarizes the cited sources. Follow each source for its full wording, notes and references.
Word-specific details from cited sources, with sources named beside each claim.
Merriam-Webster, AHD, and the Online Etymology Dictionary support back-formation from Middle English pease, reinterpreted as plural.
The Online Etymology Dictionary notes that pease could be singular or collective and that the final /s/ was mistaken for a plural marker.
Earlier forms include Old English pise (AHD also piose), from Latin pisa/pisum, from Greek pison (AHD also pisos); the Online Etymology Dictionary says the deeper Greek origin is unknown.
Merriam-Webster's 1666 date is for sense 1b “the seed of a pea,” and the Online Etymology Dictionary gives early/mid-17th century; these are recorded-use dates, not a creation date.
Pea is treated as a back-formation from Middle English pease. Back-formation: words English made by subtraction
Continue with another source-linked example.
5 history stepsschoolGreek scholē began with leisure, discussion and learning before the word reached English — while a different word entirely gave us schools of fish.scholēschool
2 history stepssandwichAn English food word is named for John Montagu, the 4th Earl of Sandwich — though the card-table story is a later anecdote.Earl of Sandwichsandwich
3 history stepsrobotA Czech word for compulsory labour became the name of a stage machine.robotarobot