From Longman Dictionary of Contemporary EnglishPollyannaPol‧ly‧an‧na /ˌpɒliˈænə $ ˌpɑː-/ noun [countable usually singular] HAPPYsomeone who is always happy and always thinks something good is going to happen
Examples from the Corpus
Pollyanna• She was a Sagittarian, open and naïve, a Pollyanna without guile or finesse who found it difficult to lie.Origin Pollyanna (1900-2000) Pollyanna, main character in the book Pollyanna (1913) by Eleanor Porter 