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 " !