웃다 is a -ㅅ verb that does not drop its ㅅ like other -ㅅ verbs, and so is one of the exceptional cases. Those other -ㅅ verbs that do, however, still behave as if an invisible consonant were there. Therefore, for all -ㅅ verbs, the -시- honorific has an emphatic vowel 으 prepended to it.
- 웃다 → 웃으십시오 (the -ㅅ is retained)
- 씻다 → 씻으십시오 (the -ㅅ is retained)
- 잇다 → 이으십시오 (not 이십시오; the -ㅅ is dropped)
- 짓다 → 지으십시오 (not 지십시오; that is a form of 지다)
- 낫다 → 나으십시오 (not 나십시오; that is a form of 나다)
Read more about the history of ㅅ-dropping verbs at this thread: https://www.duolingo.com/comment/24538573