If one has gone through a war with various stages of prolonging shock.
One who fights in battles and wars, unaware of how horrifying it really is, only to find themselves immenesly frightened; frightened by such as a clattering, a rustling, adrenaline stimuli-like head-rushing or a quickly-moving visual image or hallucination (Note, the hallucination may happen from the war anyway. Flashbacks, what have you. They are more relevant than non-war fighters.)
My great granfather was a shelder... he was under so much stress, and had so much shell shock from WWII that he ended up beating hell out of my father; even though he was an asshole to begin with and the only way to solve a problem to him was by force.
In Holden's Case, it means to "neck" with a female; kissing as some may say, or sexual activity. When Giraffes mate or "kiss" they have their butting of heads, a competition for the males to neck the other male away.
Whether it's meant by pleasing somebody or giraffes necking each other to get the female, it's all brutal.
I don't why I wrote this. So pointless. Necking! ..