This emoticon is used for when you think someone contracts herpes (the herp).
This is simply an addition to the "yeesh/yikes" emote with a MOUTH BLEMISH.
Dude A: I tongue-fucked Sarah last night.
Dude B: Sarah R_______?
Dude A: Yeah!
Dude B: She got herpes like 6 months ago from Tim Dude A: She looked disease-free.
Dude B: =/'