Without let or hindrance
'Let or hindrance' is usually used in the form 'without let or hindrance', denoting something that is free to progress. 'Without' is just a straightforward indication of a negative and isn't an example of the confusing alternative 'outside' meaning of without, as in 'without a city wall'. 'Hindrance' is also straigthforward and only has one meaning. 'Let' is the word in this phrase that causes confusion. What is a 'let' exactly?
When we talk about 'letting something happen', we are using the verb 'to let' in its most common contemporary form, meaning 'to allow'. This has been a common usage since the 10th century. Curiously, let has also been used since the 9th century to mean the exact opposite, that is, 'to hinder or stand in the way of'. It is the second of these forms that is used in 'let or hindrance'. During the 12th century, the verb was reworked into a noun and obstacles began to be called lets. That version of the word has stayed with us in the language, notably in the game of tennis, where it denotes an obstruction that is specified in the rules and prompts a point to be replayed. Such replayed points are usually the result of the ball clipping the net during a service, but a let may be any interruption to play. Streakers, who are now almost as much of a tradition at the Wimbledon tennis championships as are strawberries and cream, are taken away to the sound of the umpire gravely announcing "play a let".
Tennis is replete with obscure terms. Deuce refers to the situation where there are two points required to win a game, that is, an Anglicized version of the French 'à deux de jeu'. The origin of the term 'love' and the reason why the scores progress irregularly from 15 to 30, to 40, to game, aren't known. The suggestion that 'love' is a variant of the French 'l'oeuf', relating a zero to the ovoid shape of an egg, has no basis in fact.
Another similarly ancient sporting term for an obstacle is a 'rub', as in 'the rub of the green', so beloved of snooker commentators. A rub is more specific than a let, in that it refers to a lump on the playing surface. (See 'rub of the green' for more details on this).
The expression 'let or hinder' dates from the 16th century and is mentioned in John Baret's A Triple Dictionary, in English, Latin, and French, 1574. In the following years it began to be used in legal proceedings, with specific reference to people who hampered the police in their duties. Samuel Freeman makes a reference to such rascals in The Town Officer, 1799:
"Persons who wilfully let or hinder any sheriff or constable."
The officers who in 1974 tackled Michael O'Brien, the first known streaker at a major sporting event, might have been surprised to have been told that, in the old linguistic sense at least, they were letting him get away with it.