The story
From the triple underlining of very important votes on “the whip” — a document detailing upcoming business distributed weekly to British MPs by party managers.
Etymology from Wiktionary, CC BY-SA 4.0.
three-line whip
Today it means: A directive to MPs from party leaders to attend a parliamentary meeting and vote in a particular way, despite what the MP might believe or what his or her constituents might like. Full definition →