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The Collaborative International Dictionary
Excrescent letter

Excrescent \Ex*cres"cent\, a. [L. excresens, -entis, p. pr. of excrescere to grow out; ex out + crescere to grow. See Crescent.] Growing out in an abnormal or morbid manner or as a superfluity.

Expunge the whole, or lip the excrescent parts.
--Pope.

Excrescent letter (Philol.), a letter which has been added to a root; as, the d in alder (AS. alr) is an excrescent letter.