My standard answer when face to face with someone asking, "what's up?" I say, "A two letter word indicating general direction"
I'm afraid to ask! :0)
NOW I get it!
A talented artist named Scott Kim created a book called "Inversions" which could be viewed upside-down as well as right side up. It is very entertaining, and shows remarkable artistic talent.
A simplified version of his artistry is observable by printing the word -- up -- in lower case, large font, then turning it upside-down. It then looks like the abbreviated word down -- dn --.
Rather amusing to make a box with directional arrows and these words telling people how to orient the box.