Along with the other definitions, an Asp is a sort of telescoping club. You often see them in police dramas and suchlike; In their compact form, they are cylinders about 40mm in diameter and maybe 120mm long, although I would expect that to vary. When threatened, you take it from your pocket or wherever and shake it sharply. This makes it telescope out and lock, basically giving you a strong metal bar designed to violently beat people, which can give you an advantage in a fight. Extended, they tend to be about 600mm.
See also: ASP Tactical Baton