In this context in the US, "suk" is an informal spelling of the verb "suck". The verb "suck" has some slang uses. a. By itself , as in "You suck!" or "They suck", it's an insult that basically means "You/they are no good" or "I don't like you/them". Saying "You suck X" where X can be anything unpleasant to put into the mouth, makes the insult stronger. b. In the phrase "to suck at (doing) something" is a slang way to say "to be unable to do something well". For example, "I suck at singing" means "I can't sing". You can also say something like "you suck at singing", but it sounds rude if you do not know the listener very, very well. As a noun "a suk" is probably an informal spelling of "a suck", which is not so common among my age group, but may be more common among younger people. It is an insult, and implies that you are not a pleasant person. A related word is "a sucker", which means a naive person who is easily fooled or cheated by someone else.