The noun ending -씨 is found in several nouns and usually serves the same function of indicating "state, condition, appearance, manner." Three common words which will be looked at are: 날씨, 글씨 and 마음씨.
날씨 can be divided into the two parts 날 + 씨. "날" means "day," so combined with the ending -씨 the word means "the state or condition of the day." Hence the meaning of "weather."
글씨 is a combination of 글 ("writing") + 씨 ("state, condition, appearance"). The two parts in combination mean "the state or appearance of writing" and can be used to say that someone's handwriting is either good or bad, for example.
Finally, 마음씨 is simply 마음 ("mind, heart") + 씨. The word has a general sense of "the quality or manner of using one's heart or mind," in the sense of kindness (or lack thereof) in a person. This is close to the English sense in the translation of 마음씨가 좋다 as "kindhearted, good-hearted," as in "using one's mind or heart well."
백문식. 우리말 어원 사전. 1st ed. 서울: 도서출판 박이정, 2014. Print.