Is it ensure, insure, or assure?
"pen for pigs," Old English sti, stig "hall, pen" (in sti-fearh), from Proto-Germanic *stijan (cf. Old Norse stia "sty, kennel," Old High German stiga "pen for small cattle").
"inflamed swelling in the eyelid," 1610s, probably a back-formation from Middle English styany (as though sty on eye), mid-15c., from Old English stigend "sty," literally "riser," from present participle of stigan "go up, rise," from Proto-Germanic *stig- (see stair).
sty or stye (stī)
n. pl. sties or styes (stīz)
Inflammation of one or more sebaceous glands of an eyelid. Also called hordeolum.