The area of a man's body including (on the same
side of the body) his armpit and pectoral area in/on which a woman (or man: not that there's anything wrong with that) can rest her head and often fall asleep.
The charmpit (chest and armpit) is often utilized after
intercourse or strange sounds in the house that frighten the woman into making the man get out of bed and investigate
said frightening sounds.