A micawber is someone who does not worry about or make provisions for the future, optimistically believing that something will turn up when needed. An eternal optimist. It is a noun, so you say "he is a Micawber", not "he is micawber".
It comes from the character of Wilkins Micawber in Charles Dickens' book, David Copperfield.
Not to be confused with 'macabre'.