A posimistic person is
someone who has a very positive, affirmative,
optimistic, favorable outlook on life.
It's actually a combination of the words postive and
optimistic, hence " posimistic", "posimist " & " posimism ".
The opposite would of-course be "negamistic" , "negamist ", " negamism ", which is a combination of the words " negative " and " pessimistic " !
"Charlie is
always posimistic, expecting the best in this best of all possible worlds, but sooner rather than
later he must come to the realization that he cannot have his cake and
eat it too !"