In a way that shows scheming, plotting, or calculated intention; with cunning or deliberate purpose to deceive.
From 'designing' (present participle of 'design,' also meaning scheming or artful) + '-ly' (Old English adverb-forming suffix). The meaning evolved from simply 'intentionally' to specifically 'with cunning intent to manipulate.'
Shakespeare would have loved this word—it perfectly describes that moment when you realize someone's been playing you all along. In the 1600s-1700s, 'designing' people were those who carefully plotted their rise through society, and this adverb captured their sneaky methods.
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