I know three-line whip basically means “you must attend” or “do as you are told”. I also know that it comes from Parliament. But I never knew the derivation. Tonight I consulted my Brewers Dictionary of Phrase and Fable. A three-line whip comes from the daily agenda distributed by the party whips which lists all the votes. The important votes are “underlined three times to denote urgency and requiring party members to attend the vote.”
Failing to show up for the three-line whip is considered very serious matter. A party can withdraw the Whip from the MP, leaving him or her isolated in parliament. This more or less leaves the MP as an independent.
I like it that some words with three lines ruled beneath them are so powerful.