Weapon X was the title given to Wolverine by the Canadian government. As well as Wolverine, of course. X meaning mystery,m a hidden origin, something classified. This later became The Weapon X Project, seen in more detail in Barry Windsor-Smith's comic of the same name, and seen as being behind other mutants as well.
But in interviews. Rob Liefeld has said that he originated that idea with Deadpool. That Weapon X stood for 10 and that there were other iterations.
I told Marvel, "Wolverine is Weapon X. Weapon X is Roman numeral X. Is that correct?" They'd say, "Yes, Rob, that's correct." "To my knowledge you've not introduced Weapon one, two, three, four, five, six, seven, eight, nine. Have you done that? Do you have plans for that?" "No." I said, "Guys, he's the guy that went right before Wolverine, and they screwed him up because they had to break a lot of eggs before they made the omelet."
The thing is, there's not much mention of this in the comics. Instead, Deadpool is seen as part of a more general Weapon X project. Something Liefeld also says in the same interview,
Deadpool is the crap of the Weapon X program. And scarred, mutilated, disfigured — he ran away from the program and took his talents to use as a mercenary and bounty hunter.
Which is closer to what appears in print. Liefeld continued
And again, to further the Weapon X connection, "X-Force" #3 opens and you meet Kane, because I'm like, "Can I make Weapon XI?" So we made Kane the experiments after Wolverine. You've got Weapon IX battling Weapon XI and I'm digging my claws into the Wolverine legacy. Because that's my ultimate — I've achieved my goal. I've connected things to Wolverine.
And indeed that's not what's printed – and rather it's X-Force #2's opening…
Garrison Kane is Weapon X, not Weapon XI.
And Deadpool talks about being part of the Weapon X program, not the Weapon IX program.
As a result of this kind of thing, Liefeld's account has been dismissed by some. But yesterday, he posted his original designs for Deadpool. With notes that define the character as part of Weapon 9 – even if that never made it to the printed page…
If only they came with a verifiable date…