In a half or partial manner; incompletely or to a limited degree (archaic or dialectal).
From 'half' + '-ly' (adverbial suffix). This archaic formation directly converts the adjective 'half' into an adverb using the standard '-ly' pattern, though it's largely superseded by 'halfway' in modern English.
Halfly is English's ghost word—it's perfectly logical and was actually used, but 'halfway' eventually won out as the preferred form. This shows how even grammatically correct words can disappear when competitors are catchier or more convenient.
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