Etymologically unclear dasai has a markedly negative connotation, and is primarily used among younger people. The basic meanings are:
uncool,
boring,
nerdy,
plain
The origin of dasai is not well known. It obviously works as an i-adjective (you say ださくない (dasakunai) and you don't need na after it) but it is usually written in katakana (the i might be in hiragana), and there is no word called "dasa". Some sources say that it originated from the phrase "だって埼玉だもん", meaning "Because this is Saitama". Saitama known as a not too fashionable or interesting place as compared to the cities of Tokyo.
Another story says that dasai is a mis-reading of 田舎 (inaka). The first kanji can be pronounced "ta" or "da" and the second "sai". This is not widely believed though but it is still an interesting observation!
For more etymology, try this page.
ダサイ人
ダサイ服
ダサイホームページ
ダサイ映画
ダサイ音楽
ダサイデパート
You might say it is the opposite of 面白い.
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