So this friend of mine, she works in the Histotech lab where I hope soon to also be, and she likes to text message her buddies when they get in unusual or amusing animal parts at the lab. So this one day she texts her buddy 'ZOMG HORSE PENIS'. Ever since then, it's been kind of a catch-all phrase for those moments that blow your mind.
See a car explode? ZOMG HORSE PENIS.
Perfect stranger proposes to you? ZOMG HORSE PENIS.
100 people join your mystery knitalong? Well, I'm sure you get the idea.