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Webster 1913 Edition

Verb.

[1st pref.

un-

dull

To remove the dullness of; to clear.

[Obs.]

Whitlock.

Verb.

To remove dullness or obscurity; to clear; to purify. [Not used.]

undull ‎(comparative more undull, superlative most undull)

From un- +‎ dull

undull ‎(third-person singular simple present undulls, present participle undulling, simple past and past participle undulled)

(transitive) To remove the dullness of; clear.
(Can we find and add a quotation of Whitlock to this entry?)