a suffix occurring in words borrowed from French and other Romance languages (bassoon; balloon; dragoon; pontoon), and on this model occasionally used in the formation of new nouns in English (spittoon).
Origin of -oon
as an English formative (e.g., in spittoon), extracted from words of various origins (buffoon, lagoon, etc.) but representing chiefly French -on in words stressed on the final syllable; compare Spanish -on, Italian -one, Latin -ōn-
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
Word Origin and History for -oon
spelling conventional in 15c.-17c. English to add emphasis to borrowed French nouns ending in stressed -on; also used to represent Italian -one, Spanish ón; all from Latin -onem. Used in rare cases to form English words, e.g. spittoon, octaroon.
Online Etymology Dictionary, © 2010 Douglas Harper