unpossible

As adjectives the difference between unlikely and unpossible

is that unlikely is not likely; improbable; not to be reasonably expected while unpossible is impossible.

As an adverb unlikely

is in an improbable manner.

As a noun unlikely

is something or somebody considered unlikely.

As adjectives the difference between possible and unpossible

is that possible is (usually|not comparable) able but not certain to happen; neither inevitable nor impossible while unpossible is impossible.

As a noun possible

is a possible one.

As an adjective unpossible is

impossible.

As adjectives the difference between unpassible and unpossible

is that unpassible is (obsolete) unpassable while unpossible is impossible.