Axl is most famous for being worn by Guns N' Roses frontman Axl Rose—whose own name was a slightly less rocking Bill Bailey. It's a variant spelling of Axel, which means father of peace, though was picked by Axl in honor of one of his first bands, AXL. And apparently, Fergie and Josh picked the name in honor of that rocking Axl (though let's hope this baby avoids the more controversial aspects of his namesake's history, including drug use, domestic abuse, and mental illness).
Jack is, of course, one of the more popular names right now—currently in the top 50, though more popular than that with some people using it as a nickname for #22 boys' name, Jackson. Jack is actually a nickname for John, and means God is gracious. There's a whole slew of famous Jacks—several in nursery rhymes, author Jack Kerouac, actor Jack Nicholson—and of course, the infamous serial killer Jack the Ripper.
All in all, it's a pretty sharp name, with all those short-and-sweet consonant sounds, which makes it really stand out against the softer sounds of their surname, Duhamel.
What do you think of Fergie and Josh's choice? Too rock-n-roll for a wee baby, or something a cool kid could easily grow into?