What does un mean in the column for immigration year on the 1920 United States census? Other people have a year entered.
The "Instructions to Enumerators" for the 1920 census include the following instruction in the second paragraph, titled Definite answers:
Definite answers - Try to get a definite answer to each inquiry according to the instructions herein given. But if after every effort you can not obtain the desired information write "Un" (for unknown).