I learned this in plumbing school.

Back in the colonial days bundles of manure were put in the bottom of boats and hauled around for fertilizer for delivery. Well, as old boats do, they leak. And wet manure can create explosive gasses. Some ships hand would go below with a lantern and BOOM! No more boat. So some smart guys decided to build some shelves to put the bundles of manure on to keep it dry. They decided they should put tags on so there would be no confusion that read:

SHIP
HIGH
IN
TRANSIT

And that folks is how IT and OSHA began!