by the way, my first message I forgot to insert a period between 버릇없는/버르장머리없는 and 놈 which makes a world of difference from what I tried to explain. 버릇없다 means without any manner. 놈 is roughly equivalent to guy in english usage but it is not good to use in polite company. the attitude of mannerism differs significantly between korea and the western etiquette. so be careful of how you use words in korea if you do not want to get physical.

that 들 at the end means a group of /a bunch of. so it is probably not directed to one single individual anyway

싸가지 is a term you will hear a lot on drama series. basically reserved for someone who has no manner... unpolished, unruly. another term you might come across is 버릇없는/버르장머리없는 놈 is a common term for guy. (not really used in polite company but perfectly ok to use among close friends) In any case, 싸기지 is probably not well advised to direct at someone you don't know well and especially when you do not know what the cultural norm of dictating such use.