s

and now, for no particular reason, some obscure words begining with the letter 's':

sapid -a. flavorsome, lively, interesting (opposite of vapid, insipid)

scholarch -n. the head of a school

sempiternal -a. eternal

stridor -n. a harsh grating or creaking sound (so that's where that comes from...)

spermatoid -a. resembling sperm

sphragistics -n. the study of engraved seals

spinaceous -a. of or pertaining to spinach