A prolonged but quick "she" sound, with an "ou"-like sound from "ouch". Shee-ou.

Effectively sheow is used in place of "to laugh greatly" or like "lol" to "laugh out loud".

It's for use when an action is in progress and greater than "laughing" or "laughter" - or so I've seen it used.

It's loosely based on the Mandarin Chinese word "xiao"; which almost has a similar pronunciation and simply means "laugh".