The story
Proposed by Rev. Thomas Penyngton Kirkman in 1850.
Etymology from Wiktionary, CC BY-SA 4.0.
Kirkman's schoolgirl problem
Today it means: A problem in combinatorics, in which 15 schoolgirls go walking three abreast for seven days in succession, and must be arranged so that no two walk abreast more than once. Full definition →