I'm sure most of you have heard "necking" to mean kissing with passion; however, before "necking" the popular word among American youth was "petting". From Flappers to Rappers: The Study of American Youth Slang by Dr. Thomas Dalzell states that by the middle of the 1920s folks overwhelmingly were using the word necking instead of petting. The book doesn't state when petting was first used. Nevertheless, I'm most interested as to why folks used petting and necking to describe one who is kissing with passion. Any thoughts?
The verb 'neck' meaning "to kiss, embrace, caress" is first recorded 1825 (implied in necking) in northern England dial., from the noun.
I would imagine the implication is that the activity took place from the neck upwards.
The sense of 'petting' meaning "to stroke" is first found 1818. Slang sense of "kiss and caress" is from 1920 (implied in petting, in F. Scott Fitzgerald).
The common-sense trajectory seems to be the use of the word in relation to domestic animals, then children, then adults affectionately, then romantically.
See 'petting parties' here.
All very tame compared to bussing, it would seem. :)