Rhyming slang for "massive boner." We're talking the kind that rips your trousers and/or boxers clean off, breaks a window and pokes an old lady across the street in the eye.
(This is one of the very few rhyming slang phrases which does not actually rhyme with its counterpart.)
A: "Oh my god! I've just been hit in the face by a California redwood!"
B: "Nah, it's just some guy's Trystrem Harrison. You'll want to have a wash."
A: "Hey ladies, check out my Trystrem Harrison!"
A: "What the hell's going on!?! The front wall of your house is missing, your cat looks like it was run over by a tank, twelve lamp posts have been torn out of the ground and I've just heard that there's been an earthquake in Lithuania which registered 9.3 on the Richter scale!!"
B: "It was my Trystrem Harrison, just can't keep that bad boy under control these days."
A group of people who are commonly mistaken for "small rat like children" (see Harry Smith), due to their less than prepossessing appearance. They are in fact remarkably deft and dextrous, able to tap dance flawlessly and juggle trees. They are also extremely dangerous, able to kill a man from 500 yards in any one of 732 different ways. They can be recognised by their pretentious mannerisms, their avid dislike of all people of Mexican origin, and their aversion to Branston Pickle.
A: "Look at that man tap dancing and juggling trees."
B: "He must be a Harold Smith. Let's give him some money."
A: "Look at that small rat-like child. It's a Harold Smith."