impossible adj.

word-forming element meaning "not, opposite of, without" (also im-, il-, ir- by assimilation of -n- with following consonant, a tendency which began in later Latin), from Latin in- "not," cognate with Greek an-, Old English un-, all from PIE root *ne- "not."

In Old French and Middle English often en-, but most of these forms have not survived in Modern English, and the few that do (enemy, for instance) no longer are felt as negative. The rule of thumb in English has been to use in- with obviously Latin elements, un- with native or nativized ones.

"that may be, capable of existing, occurring, or being done," mid-14c., from Old French possible and directly from Latin possibilis "that can be done," from posse "be able" (see potent).

The only kind of object which in strict propriety of language can be called possible is the truth of a proposition ; and when a kind of thing is said to be possible, this is to be regarded as an elliptical expression, meaning that it is of such a general description that we do not know it does not exist. So an event or act is said to be possible, meaning that one would not know that it would not come to pass. But it is incorrect to use possible meaning practicable ; possible is what may be, not what can be. [Century Dictionary]