Perhaps many Japanese would be displeased when they are described as “dasai（ださい、ダサい）”.
The word spread to young people in the Kanto region in the 1970s and 1980s. At that time, the word “naui（ナウい（nowい））”, the opposite meaning of “dasai”, became so popular to use.
After the 1990s, “naui” has not been used and people think it is an obsolete and old fashioned word which is really embarrassing to use.
In contrast, “dasai” has been still used even today.
Let’s take a closer look at the word “dasai”.
What does “dasai” mean?
The meaning of “dasai” refers to the appearance of somebody, such as clothes and hairstyles, that are “not fashionable”, “unrefined”, “not neat”, “incondite”, “tasteless” and so on.
For example, a person who lives in a city like Tokyo, sometimes says “dasai” when s/he makes fun of someone from a rural area or someone who lives in a rural area. Some people compare themselves to the locals, so they feel better and they think they look better.
(In general, Japanese often have a not so good tendency to describe the countryside as not fashionable or unrefined)
Now, it simply means someone looks unfashionable, unrefined or tacky, or someone doesn’t do something very well which looks uncool.
What is the origin of “Dasai”?
There are various theories for the origin of “Dasai”.
The first is that there was a time when rural areas were called “dasha（だしゃ）,” and its pronunciation changed in conversations.
Second, the Tokyo people made fun of Saitama Prefecture as “it was the countryside of the Kanto region,” and the “sai” of “dasai” comes from Saitama.
The third theory is derived from the words spoken by boys from the motorcycle gans in the 1980s. While they were aggressively driving, there were situations where they could not change gears well which made them keep driving with inertia. The ”inertia” is pronounced “dasei（惰性（noun）” in Japanese. The word was spoken colloquially as “dassee-（だっせー、ダッセー）”, which eventually changed to “dasai” an adjective word.
The fourth theory is that it ridicules the life of Osamu Dazai（太宰治）, one of the Japanese novelists of the Meiji era.
Dazai had works inspired by the dialect of his hometown, Aomori, and his life was so different from that of the 1970s and 1980s that people at the time ridiculed the difference. Or, Dazai and “dasai” have some similar pronunciation.
(Personally I think he was just very delicate. He left a lot of great works.)
How “dasai” spread
The reason that the word “dasai” became so popular was a TV program that was so popular in the 1980s.
The program, which was broadcast at noon, was watched by young and old. The presenter was called Tamori, a comedian who was very popular at the time.
He called people from Saitama prefecture or the prefecture itself “dasaitama” in a funny manner. “DaSaitama（ダ埼玉）” was a coined word, which was a combination of “dasai” and “Saitama”. He didn’t mean any malice there. It was clear that the behavior was affectionate, from his personality. So people laughed and enjoyed the show.
At this time, Japan was benefiting from the bubble economy, and there was a fashion boom among young people, mainly around Harajuku and Shinjuku. It was the time when the DC brands were at their peak and the time when young people who lived in urban areas had great vanity and a sense of superiority.
In the era when the fashion gap between the metropolitan area and the suburbs was still large, information was limited to TV, radio, or publications. For this reason, “dasai” was a very convenient and well-targeted expression that went beyond a mere buzzword and has become established throughout the country as a slang.
Example using “dasai”
Anata no fasshion sensu wa “dasai” no hitokoto ni tsukiru.
As for your fashion sense, I can only say “dasai”.
Atarashii kuruma no dezain nandaka dasakune?
Isn’t the design of the new car somewhat clunky, right?
Watashi no jimoto ni kite kuretemo iikedo, inaka dakara dasai yo.
You can come to my hometown, but it’s just the countryside.
Watashi no kare wa sekkaku no ikemen nanoni, fuku no sensu ga dasakute zannen.
My boyfriend is very handsome, but he has bad taste in clothes which is disappointing.
Okaasan ga kattekuru fuku wa itsumo dasai.
The clothes my mom buys are always tacky.
Uchi no ani wa otaku dakara dasasa ga hanpa nai.
My brother is Otaku, so he is not interested in clothes or the like at all.
Nani sono kakkou, dasa!
What is that outfit? Not cool.
Ninki gruupu no Takuya no shifuku ga dasasugi de ukeru.
Takuya from the popular idol group whose private fashion seriously sucks that makes me laugh.