"Tarmac," a word much more popular in Britain than the US, is a shortening of tarmacadam, a word much less often used. That's a portmanteau of "tar" and "macadam." Now you might think that macadam is someone's name, and you'd be right. John Loudon McAdam was a Scottish engineer who invented macadam, which is essentially a packed gravel road.

It took humans almost a century to realize that gravel roads, though cheaper and easier than cobblestone, are garbage. Anyone who's driven down a poorly-maintained gravel road can tell you that. But don't blame Mr. McAdam too much, as the tar which binds tarmacadam together wasn't as readily available without the coal tar industry, which wasn't really as much of a thing in the early 19th Century when he was inventing his road surface. Ironically he supplied coke to Britain's first coal tar factory. He also believed that it was better for the road to drain than to hold together.

Tarmacadam was invented in 1902 by the Welsh Edgar Purnell Hooley, who must have been pretty miffed that the road wasn't called a hooley, but what can you do? Apparently he saw a stretch of macadam road where a barrel of tar had been spilled and one thing led to another. Tarmac isn't actually just tar and mac, which wasn't unknown at the time. People had tried pouring tar on macadam roads, but apparently no one had thought to mix the two and then lay the road with a steamroller. Hooley also figured out some things to add to make the road more durable.

Then the petroleum (itself a portmanteau of "petri" for "rock" and "oleo" for "oil") kicked into high gear and started producing large amounts of bitumen, which supplanted coal tar as the prime binder for macadam, and while bitumen can also be called "tar," according to Wikipedia in the UK they can use "bitmac," the portmanteau of "bitumen" and "macadam," as well as "tarmac" to refer to what many Americans would call "asphalt," a word itself derived from the Greek for natural bitumen, and still used in many parts of the world to do so.

For whatever reason, the most often used meaning of "tarmac" is to refer to paved areas of airports, most often the apron. Oddly, at least according to Wikipedia, tarmac is actually often concrete. I guess "the passengers descended to the concrete" sounds wrong somehow.