Back in the wild and wooly days of Prohibition, the story goes, crooks who robbed smuggling trucks of their loads of illegal whisky would order the drivers to "Stick 'em up high, Jack!" (or sometimes just "Up high, Jack!")

Because of this habit, they were called highjackers, then hijackers, and presto, another word entered the English language.

Another rumor has it that they pretended to be friends of the drivers, calling out "Hi, Jack!" in order to get them to stop, but I don't believe it for a minuite.