An abbreviated form of devices such as 's/he
' or 'she/he
' with the similar usage in referring to a subject of both, either, or neither gender.
It is likely a combined form of the primarily nominative 'she' and 'he' with an element borrowed from 'his' or 'him' due to their unique usage of the letter 'i'.
'Shi' and related possessive 'hir
' are seen most often in internet-presented stories involving (the usually pornographically inclined presence of) hermaphrodites or those capable of changing their sex with ease.
When shi told us who was on the phone, we knew it couldn't be good.
We took a walk, shi and I.
Shi who made us laugh nonstop with hir