Thing of the past is a term used to describe something that is no longer relevant or in use. The crossword solutions for this term are FOSSIL (6 letters), RELIC (5 letters), and ARCHAISM (8 letters). FOSSIL refers to the remains of an organism that lived in the past, such as bones or shells. RELIC is an object that has survived from the past, often associated with a particular person or event. Lastly, ARCHAISM is an old-fashioned word or phrase that is no longer in common use.