Of British-English origin:
– the noun frightener designates a member of a criminal gang who intimidates the victims of its activities;
– the phrase to put the frighteners on somebody, and its variants, mean to intimidate somebody.

The phrase to put the frighteners on somebody occurs, for example, in Lap-dancing or limp-cropping, the ‘war year’ will decide, by Stuart Reid, published in The Sydney Morning Herald (Sydney, New South Wales, Australia) of Wednesday 2^nd January 2002:

In the long term, the war against terrorism is almost by definition unwinnable. You can’t put the frighteners on a would-be suicide bomber.

The earliest occurrences that I have found of the phrase to put the frighteners on somebody are from two books by the British novelist and playwright Frank Norman (1930-1980):

1-: From Bang to Rights: An Account of Prison Life (London: Martin Secker & Warburg Ltd., 1958), Frank Norman’s memoir, in which the phrase occurs several times—for example:

One day he had a go at some little geezer who he said owed him some snout, any old how he realy put the frighteners on this geezer so much so that the geezer asked for protection, but as usual they would’nt give it to him.
[…]
One of the barons had put the frighteners on him and told him that if he did’nt pay the snout he owed him by the end of the week he would cut his ears of [sic].

2-: From Stand on Me: A True Story of Soho (London: Martin Secker & Warburg Ltd., 1960):

This geezer had some right villains on his books, and it was their job to go around putting the frighteners on the little geezers who ran the kayfs, shpielers, clubs, and dodgey book shops.

A variant of the phrase to put the frighteners on somebody occurred in the following from the Daily Herald (London, England) of Wednesday 19^th December 1962:

Judge puts ‘frightener’ on thugs
HE GIVES WARNING TO SOHO PROTECTION GANGS

THE “frighteners”—thugs who impose a reign of terror in the clubs of London’s Soho—were themselves frightened men last night.
They had heard the warning made by Judge Maude at the Old Bailey yesterday when he jailed four members of a protection-racket gang. The judge said:
‘Woe betide anyone who tries to do this sort of thing again in the heart of London. Next time the sentences will be doubled.
The courts are of firm determination to strike fear into the hearts of persons who agree to act as frighteners or thieves.’