Английский разговорный словарь (основной) - seriousü adverb (youth culture)

In young people's slang: very, truly, absolutely. Used especially in serious bad, really bad. Etymology: Formed by using the adjective serious in place of its corresponding adverb seriously, in much the same way as real had been shifted from adjective to adverb qualifying another adjective several decades previously. History and Usage: Serious used as a general intensifier, especially to qualify the adjective bad, seems to have originated among US Blacks and has been recorded in print since the mid eighties (although it almost certainly goes back further in speech). In the phrase serious bad it possibly has the function of alerting the hearer to the fact that bad is being used in its traditional or serious sense, rather than the opposite slang sense 'good' (for which see bad). With his top lip curled to signify contempt, he goaded an imaginary hapless friend: 'You a lame chief, well lame, serious lame!' New Statesman 16 Feb. 1990, p. 12 Those of you who have been popping pills and smoking dope are doing the same thing Len Bias did. Those are serious bad shots you're taking boys, serious poor judgements that you're using with your body and mind. New York Times 20 Aug. 1990, section C, p. 6