Green’s Dictionary of Slang

yelper n.

1. (also yelp) a town crier.

[UK]New Canting Dict. n.p.: yelper a Town-Cryer.
[UK]Defoe Street Robberies Considered 35: Yelp, a Cryer.
[UK]Bailey Universal Etym. Eng. Dict. [as cit. 1725].
[UK]Grose Classical Dict. of the Vulgar Tongue.
[UK]Lex. Balatronicum.
[UK]Egan Grose’s Classical Dict. of the Vulgar Tongue.

2. (UK Und.) a town clerk.

[UK]H.T. Potter New Dict. Cant (1795).

3. a wild beast.

[Scot]D. Haggart Autobiog. 62: On the Thursday evening of the races we went into the slangs, [...] seeing a conish cove ogling the yelpers.
[UK]‘An Amateur’ Real Life in London I 341: The pride of becoming an accomplished whip — to know how to turn a corner in style — tickle Snarler in the ear — cut up the yelper — take out a fly’s eye in bang-up twig.

4. a whiner, a complainer.

[UK]New Canting Dict. n.p.: yelper [...] one subject to complain, or make pitiful Lamentation for trifling Incidents.
[UK]Bailey Universal Etym. Eng. Dict. [as cit. 1725].
[UK]Grose Classical Dict. of the Vulgar Tongue.
[UK]Lex. Balatronicum.
[UK]‘An Amateur’ Real Life in London I 558–9: The Yelper† did his duty well, and finger’d the white wool* in good style. [† The Yelper — A common term given to a poor fellow subject, who makes very pitiful lamentations on the most trifling accidents].
[UK]Egan Grose’s Classical Dict. of the Vulgar Tongue.
Staffs. Jrnl 22 June 4/2: Hillo! hold hard there! [...] Springfields isn’t going to belong to such a yelper as you either.
[UK]Ipswich Jrnl 4 May 2/7: ‘Our respected’ [...] will not condescend to lift his leg at the young yelper.
[UK]E.V. Kenealy Goethe: a New Pantomime in Poetical Works 2 (1878) 336: Ass-head, Blackmoor, Cuckoo, Dotard, / Splay foot, Yelper, wry-necked Wretch.
[US]Matsell Vocabulum.
[Aus]Crowe Aus. Sl. Dict. 98: Yelper, a fellow who cries before he is hurt.

5. (US Und.) an informer.

[US]Ersine Und. and Prison Sl.
[US]Monteleone Criminal Sl. (rev. edn).
[US]Goldin et al. DAUL 242/2: Yelper. An informer.