three-line whip

Etymology
From the triple underlining of very important votes on “the whip” — a document detailing upcoming business distributed weekly to British MPs by party managers.

Noun

 * 1)  A directive to MPs from party leaders to vote in a particular way in parliament, despite what the MP might believe or what his or her constituents might like.
 * 2)  An imperative order.
 * 1)  An imperative order.
 * 1)  An imperative order.