What did Jeremy Hunt mean by "slipped" to miss a vote?

Today in the UK House of Commons, Conservative MP and PM candidate Jeremy Hunt failed to take part in an important vote. He said:

I missed votes today because I thought I was slipped and it turns out I was not.Apologies to my colleagues & Whips Office.

What does "slipped" mean in this context? It seems to be an unusual and possibly specialist meaning - I can't find any meaning in the usual dictionary sources which covers this. It might be a term from UK parliamentary jargon, or as one person on Twitter claims from UK private school slang.

Solution 1:

From "Whips and their Work":

A three-line Whip is essential and an MP frankly has no choice but to attend unless he or she has cleared their absence with their whip in advance by handing in an absence request slip with a full explanation and a pleading manner.

Hence, I presume that Jeremy Hunt believed that he was excused from attending said particular vote having completed an absence request slip (or having had one of his staff complete the slip for him).

For non-UK readers, a "Whip" is an MP (Member of Parliament) charged with ensuring that fellow MPs from the same political party attend certain votes. The most important votes will be underlined three times on the weekly sheets distributed to MPs, and are hence called "three-line whips". The term originates in hunting, where a "whipper-in" is "a huntsman's assistant who keeps the hounds from straying by driving them back with the whip into the main body of the pack" (from OED).