It is British military slang, which is widely used on civvy street as well.
In fact, I would say that most educated adult Brits know what a Walt is.
Am I a Walt?
As an ex member of the British army, who served in 2 theaters of war, I find that question very offensive.
Is that how you usually talk to veterans, questioning their service while hiding behind a keyboard?
Remind me again, where did you serve Trannie?