What is the meaning of the phrase "Entertaining Angels"? For inexplicable reasons I've been listening to a lot of Newsboys lately and also recently finished reading Dorothy Day's Auto-Biography.  What do these things have in common?  Well there's an intensely catchy Newsboys song called "Entertaining Angels" and the biopic of the life of Dorothy Day is also called the same thing.  

Since I'd never heard the phrase before I figured I'd ask about it here so I could impress my wife later on. Is it an old-fashioned sort of saying or a Biblical allusion (Gen 18) or something else?  Does it mean Angels are laughing at us foolish mortals or we should treat everyone we meet as if they're angels in disguise?
It's a phrase from Hebrews 13:2:


  Do not neglect to show hospitality to strangers, for thereby some have entertained angels unawares. (ESV)


Most commentators do think that it refers to the Abraham story, as well as Lot, Gideon, and Samson's parents. See this BH question for more.