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
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-
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.