It could be a contraction of "il y a", which means "there is/there are" or "ago". For example:
* "Il y a 5 minutes" (There are 5 minutes) could be shortened to "Y'a 5 minutes" or even "Lu 5 minutes".
It could also be a shortened version of "lui", which means "him" or "her" in French.
However, without context, it's impossible to determine the exact meaning.
Could you please provide more context for the word "lu"?