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Webster 1913 Edition

(sĕl′tĭ-sĭz’m)

Noun.

A custom of the Celts, or an idiom of their language.

Warton.

Noun.

The manners and customs of the Celts.

Celticism ‎(plural Celticisms)

A Celtic custom or idiom.
(Can we find and add a quotation of Warton to this entry?)