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-
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Online Etymology Dictionary, © 2010 Douglas Harper