So, you know how sometimes you might want to be really good at something and you feel jealous of other people who seem to be better at it than you? Well, scientists sometimes feel the same way about different branches of science.
Physics envy is when scientists in other fields, like biology or psychology, feel like physics is the coolest and most important field of science. They might think that physicists have all the fun and get to make the biggest discoveries. They might even try to use ideas from physics in their own work, even if it doesn't really fit.
But just like how everyone has their own strengths and talents, each branch of science has its own important questions and ways of studying them. Physics might be great at studying the basic laws of the universe and how everything moves, but biology and psychology are important for understanding living creatures and how they think and feel.
So, in short, physics envy is when scientists in other fields look up to physics and feel jealous of its coolness, but they should remember that their own fields are just as important in their own ways.