What's the meaning of the phrase 'Chow down'?
To chow down is to sit down to eat - similar in meaning to the British term 'tuck in'.
What's the origin of the phrase 'Chow down'?
A chow is a breed of Chinese dog. The reputation that poor Chinese had for eating dogs has led to some people believing that was the origin of chow being used as a slang term for food. In fact that's not correct - the use of 'chow' to mean food pre-dates the naming of the chow breed of dog. Nevertheless, that reputation for eating animals which are regarded in western countries only as pets also resulted in the Australian slang term for Chinese food - 'chow miaow' (recorded since at least 1948).
'Chow', in the sense of food, is recorded in English from the mid-19th century. In Spirit of Age, 1856 we get this line:
"Ah Chow - ah in the Celestial lingo means Mr. Chow something good to eat."
Chow actually derived as the Anglicised version of the Chinese word for 'stir fry'.
By the late 19th century the word 'chow' had travelled from China to India and thence to the west. In Sir Henry Yule and Arthur Burnell's Hobson-Jobson: being a glossary of Anglo-Indian colloquial words and phrases, 1886 were unambiguous in their definition:
"Chow is in 'pigeon' applied to food of any kind."
That 'pigeon' isn't a reference to the bird. Pidgin English was the name the English gave to the language that the inhabitants they came across in the lands of their imperial conquests. This language was often a hybrid of the native language and English, French and Spanish.
The phrase 'chow down' originated in the U.S. military around WWII. The earliest printed citation I can find is from The Hammond Times, December 1942, in a story about life on a submarine:
"'Chow down, sir' a Negro mess attendant in a white coat informed.... "Served on navy blue and white china we put away steak, potatoes, peas and ice cream."
It appears that the original meaning of 'chow down' was literal, that is, it referred to swallowing food down one's throat. That usage is recorded slightly earlier that 'chow down' itself - in the US newspaper The Wilkes-Barre Times Leader, September 1937:
You can ask me anything, but only after Williams and I have thrown some chow down our throats.
The most common use of the word now is in 'chow mein' - the dish made from noodles, sauce and meat/vegetables. This is first referred to in print in the early 20th century. 'Mein' is the Anglicised Chinese word meaning noodles and, as 'chow' means 'stir-fry', 'chow mein' is exactly what you might expect - stir fried noodles.