Petting Meaning

Here is the actual sentence from the "College Slang" sidebar in chapter 2 ("The 1920s: The Flapper") of Tom Dalzell, Flappers 2 Rappers: American Youth Slang (1996) in which necking and petting are discussed:

necking replaced "petting" to mean kissing, while a party referred either to a girl who necked or necking itself.

In the book's glossary of 1920s slang, Dalzell includes this entry for necker:

necker A petter who puts her arms around a boy's neck

And this one for petting pantry:

petting pantry A movie theater

Dalzell revisits necking and petting in chapter 4 ("The 1940s: The Jive Generation"), though with very little explanation of what it meant:

What was in the past known as petting or necking was known by the sub-deb as boodling, gooing it, hacking, monking, mousing, mugging, or smooching.

I have detailed elsewhere at some length my unsuccessful attempts to corroborate another of Dalzell's assertions about 1920s slang—namely, that grungy meant "envious"—and in a comment above, FumbleFingers alludes to another dubious claim about 1920s slang in the same work: that pine feathers period referred to "The period in a Flapper's life when she blossoms out."

It is certainly possible that some young people in the 1920s used petting in the narrow sense of "kissing passionately" (perhaps further distinguished by the girl's having her arms around the boy's neck), but I wouldn't reach that conclusion on the strength of Dalzell's assertions alone. I also recommend being extremely cautious about using contemporary online sources to corroborate Dalzell, since many such sources base their information on Flappers 2 Rappers, and therefore do not provide independent confirmation of its conclusions.