Monty Python and the Holy Grail (1975)

Continuity mistake: In the scene where Sir Galahad is about to enter Castle Anthrax, it's pouring like there's no tomorrow. Although when he falls against the door and is about to open there is no rain, and no Niagara Falls pouring down the door. As a matter of fact, it's pretty dry on the door and almost a completely clear sky.

Second brother: And Saint Attila raised the hand grenade up on high, saying, 'O Lord, bless this thy hand grenade that with it thou mayest blow thine enemies to tiny bits, in thy mercy.' And the Lord did grin, and the people did feast upon the lambs and sloths and carp and anchovies and orangutans and breakfast cereals and fruit bats and large chu...
Maynard: Skip a bit, Brother.
Second brother: And the Lord spake, saying, 'First shalt thou take out the Holy Pin. Then, shalt thou count to three, no more, no less. Three shalt be the number thou shalt count, and the number of the counting shall be three. Four shalt thou not count, nor either count thou two, excepting that thou then proceed to three. Five is right out. Once the number three, being the third number, be reached, then lobbest thou thy Holy Hand Grenade of Antioch towards thy foe, who, being naughty in my sight, shall snuff it.'
Maynard: Amen.
Knights: Amen.
King Arthur: Right! One... Two... Five!
Sir Galahad: Three, sir!
King Arthur: Three!

Trivia: The idea for the killer rabbit came from the facade of the Cathedral of Notre Dame. Near the entrance, in panels depicting various scenes of man's infirmaties, one panel illustrates cowardice by showing a knight fleeing from a rabbit.

Question: During the song "Knights of the Round Table" they sing the line: "Between our quests we sequin vests". What does this mean? (English is not my mother tongue.)