What's the "een" in Halloween?
early or mid-17c., false singular from Middle English pease (plural pesen), which was both single and collective (e.g. wheat, corn) but the "s" sound was mistaken for the plural inflection. From Old English pise (West Saxon), piose (Mercian) "pea," from Late Latin pisa, variant of Latin pisum "pea," from Greek pison "the pea," perhaps of Thracian or Phrygian origin [Klein].
In Southern U.S. and the Caribbean, used of other legumes as well. Pea soup is first recorded 1711 (pease-soup); applied to London fogs since at least 1849. Pea-shooter attested from 1803.