Verb:
1)Somthing you look up in Urban Dictionary when your uncreative brain isn't able to figure out a better definition to look up. As "look up" is written next to the look-up-box.
2) Something you did a few seconds ago,of course after giving this definition a thumb up.

antonym: look down

What it usualy says before a search box on a dictionary website (on the other side from the search button)

To search online, in a book, magazine, dictionary, encyclopedia, other writing work, for an answer or other need

i am good and fun

to look up

to look up as in to look into the sky retard.