The present participle of frontispiece when used as a verb; the act of adding or creating a frontispiece for a book or building.
From 'frontispiece' functioning as a verb + '-ing' gerund/present participle marker. This is a rare bibliographic or architectural term.
In historical printing, 'frontispiecing' was an actual profession—specialized artisans would create custom engravings for wealthy patrons' books, making this 'rare' word a window into extinct trades.
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