Lipstick index is a fancy term for something very simple - it's the idea that when times are tough, people still like to treat themselves. One way that people do this is by buying small things that make them feel good, like a tube of lipstick.
When there's a recession or a difficult economic period, people might not be able to afford big purchases like cars or expensive vacations. But they might still want to have a little treat that makes them feel happy and confident. So they might buy a colorful lipstick or a fancy lipstick brand to make themselves feel good.
This term was first started by a famous investment banker called Leonard Lauder in the 2000s. He noticed that during recessions, sales of lipstick went up, even when people weren't buying a lot of other things. This has led some people to refer to lipstick as an "economic indicator" because it can give us a clue about how people are feeling about the economy.
So, in summary, the lipstick index is the idea that people might still treat themselves to small items like lipstick even when the economy is tough, as it can make them feel good and is a way to indulge themselves without breaking the bank.