Belittling and often insulting term for being absurdly optimistic and good-hearted, believing in a good world where everything works out for the best all tht time. Often in combination with being God-fearing and perceiving oneself standing on a higher moral ground than others.
See also: Daddy's Little Girl, Goody Two-Shoes
The word "Pollyannaish" is derived from "Pollyanna," the heroine of Eleanor H. Porter's famous novel "Pollyanna" (1913), whose outlook on life was one of absolute optimism and whose problems were always straightened out in the end.
(Disclaimer: Not very modern, but still an urban term.)