Well, the ending (어)서 means "because, because of". You can add it at the end of a verb. In the case of 셔서, you're using the honorific particle 시 + 어서, so the contraction becomes 셔서: With no honorific: 추워서 안 갔어요 It was cold, so I didn't go. (춥다 + 어서) With honorific: 어머니 시내에 가셔서 저는 저녁 식사를 준비하고 있어요. Because my mother went to town I'm making dinner (가다 + 시 + 어서)

i think "셔서" is not a noun. maybe "셔서" is a term of respect. for example, 할아버지는 아프셔서 못 오셨어요. translate english. Grand father does not come here because he is sick. "셔서" is not important in sentence. that is honorific.