Green’s Dictionary of Slang

1. a soft-hearted, pious person.

1831		Navy at Home II 238: I’ve heard as he’s [i.e. a ship’s captain] turned psalm-singer.
1838		Dickens Oliver Twist (1966) 164: They’re soft-hearted psalm-singers, or they wouldn’t have taken him in at all.
1899		‘Pot’ & ‘Swears’ Scarlet City 189: The stingy old psalm-singer was done out of his supper.

2. (US prison) an informer, a prison trusty.

1910		J. Sullivan ‘Criminal Sl.’ in Amer. Law Rev. LII (1918) 891: A ‘prison stool pigeon’ is a ‘trusty,’ ‘psalm singer’ or ‘pig’.
1949		Monteleone Criminal Sl. (rev. edn).

(US) a clergyman.

1946

J. Evans Halo in Blood (1988) 16: I never – not once, Mac – been to one where there was more’n one psalm-slinger to say the words over the stiff.

a non-conformist, a street preacher.

1843		Taunton Courier 15 Feb. 5/4: A cordwainer by trade [...] but an indifferent ‘psalm-smiter’.
1846		Western Times 24 Oct. 3/1: ‘That’s a psalm-smiter’ (exclaimed one of the officers of the court) I’ll be sworn!’.
c.1850		Peeping Tom (London) 35 138/1: Some fluent and right rascally ‘psalm-smiter,’ as he himself was wont to term such canting fellows.
1860		Hotten Dict. of Modern Sl. etc. (2nd edn) 194: psalm-smiter a ‘Ranter,’ one who sings at a conventicle.
1873		Sl. Dict.
1882		Cremorne I 25: The man you jilted for a psalm-smiting sneak.
1882		Sydney Sl. Dict. (2 edn) 11: Psalm-smiter - An itinerant preacher.
1889		London & Provincial Entr’acte 16 Oct. 4/2: I hope the Bishop of London will be called a a witness. His Lordship was present when the reverend psalm-smither made the [...] charge.
1896		Leighton Buzzard Obs. 10 Mar. 5/3: The letter was [...] characterised [...] as a ‘tissue of falsehoods,’ while the writer described as a ‘psalm-smiter’.
1910		Bulletin (Sydney) 29 Sept. 10/1: The most unconsciously-humorous man in Melbourne is a converted psalm-smiter who lately referred to Sam Mauger as a ‘wowser.’ The unconscious humorist, it seems, was formerly a devoted Calathumpian with only one failure.

(Aus.) religiosity.

1885

Bulletin (Sydney) 31 Jan. 20/1: Run him in, too, for in spite of all his craw-thumping and Psalm-snuffling deeds, his pile is made up of ‘crime and immorality,’ and he is just as great a vagrant as the veriest ‘bummer’ or magsman in the land.