你老竇個仔, is it a minced oath in this case? If so what would be the original meaning?
Yes, it is. Cantonese swearing can be very vicious and graphic sometimes.

The most common Cantonese swearing is 「我(F)你老母」and one of a variants is 「我 (F) 你老母個 臭化(C)」; 臭化 means smelly and rotten. 


  「你老竇個仔」 is a minced oath for 「你老母個 (C) 」, short for 「我(F)你老母個 (C)」


Replacing "老母" (mother) with "老竇" (father), and replacing the Cantonese "C" word with a similar sounding word "仔"

To translate it into English, you can use any minced oat, as long as it expresses strong disapproval. 

For example: "Gat Dangit" --> "God damn it"