completionist adjective 1noun 1forms 3 adjective1 •Engaging in completionism.M did not have this completionist fetish regarding other matters—he would happily leave a meal uneaten and a book half read—but in matters of travel it was something of a matter of pride. — 2016 noun1 •A person who practices completionism.If you are a Halo completionist and need to experience every moment of the Halo story, you have no choice. — 2009 formscompletionists · more completionist · most completionist