Definition
Tragic optimism means being optimistic in spite of the tragic triad. It is not a naive belief that “everything will be fine”, but the conviction that in every situation - even a tragic one - a person can find meaning and give the best answer.
Source
Frankl developed the idea in the essay “The Case for a Tragic Optimism” (1984), added as a postscript to later editions of “Man's Search for Meaning”.
Optimism cannot be commanded
Frankl warned that optimism and happiness cannot be decreed. A person needs a reason to be happy - and that reason is meaning. Once it is found, joy comes by itself.
Sources
V. E. Frankl, Man's Search for Meaning (postscript, 1984); Wikipedia: Logotherapy.