Before SL 1.2 was released, SL had a completely different resource allocation system. There were only 10,000 prims per sim, (as opposed to the current 15,000), and they were shared. In theory, all 10,000 prims could be put on a single 16 sq. m. plot of land. This led to what is known as the Tragedy of the Commons, where some residents took a larger share than what was deemed "fair" by the rest of the community, generating friction. This newfound scarcity led to what residents called "prim banks", where a resident would rez prims specifically so that others couldn't have them. This ensured that said residents would be able to expand their build in the future, by deleting those prims from the "bank".
Interesting tidbit: The first actual "prim bank" was conceived in Kissling in July, 2003. The owner of the property made an actual vault and placed each prim modeled as a gold bar. After a griefing war with neighbors, and Linden intervention, the intentional prim bank was removed.
A prim slut is a resident who (ab)uses/wears too many prims--particularly in hair (even worse with flexible prims) and jewelry (which usually has lots of bling). Before version 1.10, SL's local lighting tended to cause a lot of lag--especially on high-prim attachments worn by prim sluts (who usually had local light disabled).