An individual who exhibits agitation, revulsion, panic, or extreme discomfort for no worthwhile reason. Akin to the medical disorder "sandvaginitis", or "appearing to have sand in one's vagina". (Note: this term applies to female as well as male individuals).
Paco: Miguel, stop being such a sandvag. A bite from the death stalker scorpion causes unbearable pain, fever, coma, convulsions, paralysis and only maybe death.
Miguel: Is Raul going to come over and suck the poison out of my wound?
Paco: If he doesn't sandvag out this time.