Definition of Irish moss
1
: the dried and bleached parts of a red alga (especially Chondrus crispus) used as an agent for thickening or emulsifying or as a demulcent and as a source of carrageenan
2
: a red alga that is a source of Irish moss
— called also carrageen
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Merriam-Webster unabridged
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Time Traveler for Irish moss
The first known use of Irish moss was in 1837
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