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 " !