sss, ssssss, ssssssssssssss etc. with as many 's's as desired.

  1. the hiss of a snake or other reptile (can be any multiple of the letter s)

The above text is a snippet from Wiktionary: sss
and as such is available under the Creative Commons Attribution/Share-Alike License.

Need help with a clue?
Try your search in the crossword dictionary!