Relating to or of the nature of an excrescence; characterized by abnormal growth or being superfluous.
From excrescence with the -ial suffix (meaning 'relating to'), creating an adjective form that describes something as relating to or involving abnormal growths.
This is an even fancier version of 'excrescent'—while linguists might say a sound is 'excrescent,' they might use 'excrescential' when discussing the phenomenon of excrescence itself in a technical paper.
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