Kinagashi (dressing kimono casually) is a style in which a man doesn't put on hakama (pleated and divided skirt made in fine stripes) when he wears wafuku (Japanese traditional clothes). Sometimes it also means how to dress in that style.
In days gone by, only the style in which one omitted haori (a Japanese half-coat) was particularly called kinagashi, and if only he had haori on, he could be considered to be in a formal costume even if he didn't have hakama on, but now kinagashi often indicate the style without hakama, whether he had haori on or not. In the Edo period, while the samurai warriors always put on hakama, the commoners in towns didn't have that convention, and therefore, kinagashi was thought to be a custom peculiar to the townspeople.
Generally speaking, kinagashi is considered to be an informal and casual style, and especially it is thought that the costume in which haori is omitted shows a unique aesthetic sense which is common to iki (粋, traditional aesthetic ideal in Japan like nattiness) and tsu (通, highly cultivated person in Japan like a connoisseur), and high spirits and vulgar impressions which illustrate characteristics of the lower classes. However, there are also some cases where the commoners in towns as such have an original awareness of kinagashi, and they treat it as a formal dress: for example, in Sado (Japanese tea ceremony) of the Sen Family line, they think that kinagashi with jittoku (a kind of haori) on is the utmost formal dress; and in the case of nagauta (long epic song with shamisen accompaniment) or joruri (dramatic narrative chanted to shamisen accompaniment), when they wear kamishimo (samurai costume, old ceremonial costume) they don't put on hakama but maedare (an apron). Generally even in the Edo period, kinagashi with jittoku on is often considered to be a formal attire for doctors, sado (a person in charge of the tea ceremony), chabozu (tea-server), hired commoners in towns, and so on, which is thought to be based on the lifestyle of the commoners in towns in the Muromachi period.
Mawarikata-doshin (patrolling police constables) of Edo Kitamatchi-bugyo (one of two town magistrates) are known to have worn kinagashi of "Onarisaki-gomen" which exempted them from putting on hakama at the place Shogun visited (this was the privilege granted only to them.)