In a way that is expressible or can be expressed; with capacity for expression.
From 'expressible' plus the adverb-forming suffix '-ly'. The chain goes: Latin 'exprimere' → 'express' → 'expressible' → 'expressibly'.
This adverb is so rare that it almost never appears in modern writing—most speakers would just say 'expressly' or 'clearly' instead. It shows how English sometimes creates valid words that nobody uses.
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