This word is used widely by Malaysians and also Singaporeans. This word is also known as one of the most super negative word. It is not a swear word, believe it or not. It's an expression of a combination of multiple negative feelings as follows:-
bored, fed up, sick of it, soured, bitter, headache, monotonous and any negative feelings you can think of.
"Everyday, doing the same thing over and over again. I'm so damn sien ar!"
"I hate to do this report. Everytime I think of it, I'm so sien"