Playful trickery, tomfoolery, or the antics characteristic of a harlequin; buffoonish behavior.
From harlequin plus the suffix -ery (denoting action, practice, or style), creating a noun meaning 'the behavior or style of a harlequin.' This follows the pattern of other -ery nouns like trickery or buffoonery.
The suffix -ery is one of English's most useful word-building tools—it turns nearly any noun into a concept representing 'the quality of being that thing,' which is why a single trickster character name gives us harlequinary, harlequinade, AND harlequinery.
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