As the story goes, the word ‘Bistro’ entered the French language after the Battle of Paris (1814), when hungry Russian officers – who had just conquered Paris – would shout at waiters ‘bystro’, which means quickly, or hurry up, in Russian.
In time, it came to stand for small french restaurants serving, quickly, slow-cooked foods that were made in advance.
Typically, bistros offer a moderate-priced menu in a modestly designed setting.
ps.
When in Paris be sure to check out the sign at 6 Place du Tertre in old Montmartre (see picture below). It marks the spot where ‘Bistro’ was first coined.

pps.
Some new etymologists suggest that ‘Bistro’s origin is the word ‘bistrouille’ which describes brandy mixed in coffee, but I definitely stick with the old school version.