impossibly comes from impossible (from Latin impossibilis, 'not possible', from in- 'not' + posse 'to be able') + the adverbial suffix -ly. Morphologically, the ending is part of the etymology: the suffix is attached to the older base named in the history, producing a later English derivative. In present English, the adverb sense is "to a degree that is impossible or seemingly beyond belief", so the historical development ends in a clearly identifiable modern dictionary use. The same base also appears in related forms such as impossible, impossibility, and possible. Its semantic neighborhood includes unbelievably, incredibly, inconceivably, and unfeasibly, which helps locate the modern sense without adding a separate etymological claim. The named historical forms include Latin impossibilis and Latin posse.