This, of course, is a classic line from Monty Python and the Holy Grail. It appears in the movie's first scene, in which King Arthur and his trusty servant Patsy approach a castle and are questioned as to where they found the coconuts they use to mimic the noise of their imaginary horses. King Arthur tries to move off the subject, but the castle guards begin a conversation as to whether a swallow could carry a coconut.
Soldier #1: Where'd you get the coconuts?
Arthur: We found them.
Soldier #1: Found them? In Mercia? The coconut's tropical!
Arthur: What do you mean?
Soldier #1: Well, this is a temperate zone.
Arthur: The swallow may fly south with the sun or the house martin or the plover may seek warmer climes in winter, yet these are not strangers to our land?
Soldier #1: (pause) Are you suggesting coconuts migrate?
Arthur: Not at all. They could be carried.
Soldier #1: What? A swallow carrying a coconut?
Arthur: It could grip it by the husk!
Soldier #1: It's not a question of where 'e grips it! It's a simple question of weight ratios! A five ounce bird could not carry a one pound coconut.
Arthur: Well, it doesn't matter. Will you go and tell your master that Arthur from the Court of Camelot is here?
Soldier #1: Listen. In order to maintain air-speed velocity, a swallow needs to beat its wings forty-three times every second, right?
Soldier #1: Am I right?
Arthur: I'm not interested!
Soldier #2: It could be carried by an African swallow!
Soldier #1: Oh, yeah, an African swallow maybe, but not a European swallow. That's my point.
This opening scene isn't explained, and left to linger on the minds of the confused and amused moviegoers, until Scene 23, near the end of the movie. King Arthur and his loyal Knights are faced with answering three questions by the Keeper of the Bridge of Death, lest they be cast into the Gorge of Eternal Peril. After Sir Launcelot successfully answers the three questions and crosses the Bridge, Sir Robin approaches and is stumped by the third question (What is the capital of Assyria?) and ends up in the Gorge. So does Sir Galahad, who couldn't decide whether his favorite color is blue or yellow (it was yellow), so King Arthur and Sir Bedevere approach the Bridgekeeper.
Bridgekeeper: Hee hee hee. Stop! What... is your name?
Arthur: It is Arthur, King of the Britons.
Bridgekeeper: What... is your quest?
Arthur: To seek the Holy Grail.
Bridgekeeper: What... is the air-speed velocity of an unladen swallow?
Arthur: What do you mean? An African or European swallow?
Bridgekeeper: Huh? I — I don't know that! Auuuuuuuugh!
Bedevere: How do you know so much about swallows?
Arthur: Well, you have to know these things when you're a king, you know.
Why do I bring all this up? One day, after watching Holy Grail for the eleventy-first time, I began wondering what the air-speed velocity of an unladen swallow actually was. This was around the time that Ask Jeeves (www.ask.com) was getting popular, so I decided to see if Jeeves knew the answer.
Greetings. Please enter your search or question
What is the air-speed velocity of an unladen swallow? [enter]
Do you want to know: What is the air-speed velocity of an unladen swallow? Ask [click]
What do you mean? An African or European swallow? Ask
If you then click on the Ask link to find out what type of swallow you mean, you're taken to the scripts of the swallow scenes from Monty Python and the Holy Grail!
Also, for the curious, Deep Thought (an SCO UNIX system in Santa Cruz) placed this question among other signup-related questions, and logged the answers it received. The list can be found at http://www.armory.com/swallow.html. My favorite answer: "what isn't it laden with?"
For those of you who've had enough of this silliness and want a real number, skongshoj says "The maximum air-speed velocity of the common barn swallow (European and American, I don't think they live in Africa) is about 45 mph." sam512 says "...a swallow generally flies at around 40-50mph." So, there you go.
Movie quotes taken from http://www.armory.com/swallowscenes.html#question. Monty Python and the Holy Grail is copyright 1974 Python (Monty) Pictures, Ltd. Quotes used are within the limits of fair use.