Also known as a "walking stick shotgun," a poacher's stick is a highly cool, but also highly illegal trinket that resembles a fairly standard walking stick or cane on the outside, but is, in fact, a 12-bore shotgun disguised to resemble a walking stick. It has the barrel throughout its length, and the trigger hidden underneath the crook of the stick.
The reason why it's called a "poacher's stick" is because you can be out walking in the woods on some large estate, when all of a sudden a humongously fat and juicy, plump-breasted pheasant presents itself before you. With a quick check in all directions to check nobody's looking, you aim your stick in the direction of your quarry, and blast it to death through the bottom of the stick, which forms the aperture. The butt of the stick can be changed to allow a full choke, half choke or true barrel as appropriate, incidentally.
So, having bagged your ill-gotten gains and stuffed them into your inside coat pocket, you continue walking along as if nothing had ever happened. After all, you're just walking in the woods with a stick for comfort, so when the wardens appear and ask you if you heard a shot near you, you can point them in some other direction not too far off and carry on. And spy another plump-breasted pheasant and repeat.
Poacher's sticks are, in the UK, considered concealed weapons and are thus by definition completely illegal. That being said, though, the Criminal Justice Act 1988 provides an exception for a museum or gallery's possession and exposure of any offensive weapon:
"8) It shall be a defence for any person charged in respect of any conduct of his relating to a weapon to which this section applies —
(a) with an offence under subsection (1) above; or
(b) with an offence under section 50(2) or (3) of the Customs and Excise Management Act 1979,
to prove that the conduct in question was only for the purposes of making the weapon available to a museum or gallery to which this subsection applies.