Green’s Dictionary of Slang

1. (orig. boxing jargon) a knockout blow; thus also, a professional fighter/boxer, i.e. one who ‘clicks’ his opponent.

1821		‘Battle’ in Fancy I XIX 449: A match for 20 guineas a side between Gould a navigator, and Harris the Blackwall clicker, was the principal attraction.
1859		Matsell Vocabulum 19: clicker A knock down.
1881		Trumble Sl. Dict. (1890).

2. a watch.

1819		Beppo in London x: He treated them with liquor, And paid the score by pawning of his clicker.
1821		‘An Amateur’ Real Life in London I 619: Tallyho was secretly eased of his clicker.* [*Clicker—A flash term given to a watch].
c.1850		Duncombe New and Improved Flash Dict.

3. a telegraph operator.

1894

Mirror of Life 7 Apr. 13/1: ‘Nobody ever heard of a telegraph operator getting rich,’ interrupted a young man who was getting 50 dols a month as a clicker at the railroad station.

4. (UK prison) a warder.

1925

H. Leverage ‘Dict. Und.’ in Flynn’s mag. cited in Partridge DU (1949).

5. (US) an amateur photographer.

1940

R. Johnston ‘Slanguage of Amateur Photographers’ AS XV:4 358/1: Clicker. An amateur photographer.

6. see click n.⁴ (3)