1) An archaic term for a room within a home used for recreational indoor activities or the entertainment of guests; known as a "living room" in modern language.
2) A room equipped and furnished for a special function within a business. (i.e. Ice-cream parlor, tanning parlor, gambling parlor, pool parlor)
3) Slang for a brothel or place of prostitution, or a hall or den used in secret to consume narcotics & illegal substances.
"Julius, why don't you show the guests to the parlor and bring them some o' my fine Kentucky whiskey?"
"Oh golly! Mr. Whipple down at the ice-cream parlor sure makes a swell milkshake!"
"The parlor got raided by the fuzz; now where'm I gonna get a fix?"