Green’s Dictionary of Slang

1. (US) an act of scrutiny or staring.

1914

Jackson & Hellyer Vocab. Criminal Sl. 61: necking [...] A scrutiny: an impertinent staring [...] Example: ‘The guinea on the end is giving you a necking through the glass.’.

2. kissing and cuddling.

1924		P. Marks Plastic Age 28: The hero was given careful instructions. ‘Some neckin’, Harold!’.
1934		(con. 1920s) J.T. Farrell Young Manhood in Studs Lonigan (1936) 421: You don’t want to be riding around in automobiles with fast young fellows, petting and necking.
1942		P. Wylie Generation of Vipers 64: The automobile is, for them, a detached, portable bedroom. In it they consummate their sex necessities by what they call ‘petting’ – still naively assumed, by older folk, to be synonymous with ‘necking’.
1952		Lait & Mortimer USA Confidential 162: [We] saw B-girls in each of these places—necking or worse in corners with servicemen.
1965		E. Bond Saved Scene x: Why did the policewoman marry the ’angman? [...] They both liked necking.
1977		(con. 1950s) Nicholson & Smith Spend, Spend, Spend (1978) 46: I knew something might come of it like a good necking session.
1998		‘Bill E. Goodhead’ Nubile Treat 🌐 I thought Cecilia was ready ... for some heavy necking. We were in the car, parked up by the cliff, and I ... I guess I got a little out of line.

3. (US Und.) hanging.

1949

Monteleone Criminal Sl. (rev. edn).

4. (Aus.) a means of pickpocketing whereby the thief puts one arm around his victim’s neck and the other into his pocket.

1968

J. Alard He who Shoots Last 173: All his boob dreams of performing future neckings (his favourite method of earning a living) were fading.