UBTREAS is part of a broken sign that once said SUBTREASURY. John encounters it when he's exploring New York City in Stephen Vincent Benét's "By the Waters of Babylon."
When John ventures into New York City, he finds the broken remnants of buildings, though some have remained intact. He finds the intact ones hollow and abandoned. The sight of the city is overwhelming to the man who grew up with the Hill People, forbidden to travel through the Dead Places.
As John explores NYC, he thinks:
How shall I tell what I saw? The towers are not all broken—here and there one still stands, like a great tree in a forest, and the birds nest high. But the towers themselves look blind, for the gods are gone. I saw a fishhawk, catching fish in the river. I saw a little dance of white butterflies over a great heap of broken stones and columns. I went there and looked about me—there was a carved stone with cut—letters, broken in half. I can read letters but I could not understand these. They said UBTREAS. There was also the shattered image of a man or a god. It had been made of white stone and he wore his hair tied back like a woman's. His name was ASHING, as I read on the cracked half of a stone. I thought it wise to pray to ASHING, though I do not know that god.
UBTREAS is the sign from the Subtreasury building. The building that once served as the subtreasury in New York City is called Federal Hall, and there is a statue of George Washington out front. The building is where George Washington took his oath of office, according to the National Parks Service.
After John sees the wonders of New York, he returns home to his tribe, determined to use what he's learned to begin to build civilization again when he becomes the chief priest.