bestofwikipedia:

Genital retraction syndrome, generally considered a culture-specific syndrome, is a condition in which an individual is overcome with the belief that his/her external genitals—or also, in females, breasts—are retracting into the body, shrinking, or in some male cases, may be imminently removed or disappear. A penis panic is a mass hysteria event or panic in which male members of a population suddenly experience this belief. (via Sasha)

I like the term “penis-snatchers” in the article.