The story
From semicentenary (“50 year, 50th anniversary”) + -ian (“person”), equivalent to semi- (“half”) + centenarian (“100-year-old”), q.v.
Etymology from Wiktionary, CC BY-SA 4.0.
semicentenarian
Today it means: Synonym of fiftysomething: A person celebrating a semicentenary; a person between 50 and 59 years old. Full definition →