Nostalgie de la boue generally has a sexual connotation, which is one of the reasons the words popped into my head when I thought of the my student days. Although in my case, I suppose my nostalgie could also be categorized as “Nostalgie de mon jeunesse”.
The phrase also popped into my head because I see the word boue every time I drive down the mountain to Carcassonne. There is a warning sign by the road that says “Boue après Orage” We finally figured out it must mean that we should watch out for mud on the road after a storm. The sign prompted a number of interesting discussions about nostalgie de la boue and exactly what kinds of sex acts and with whom constitute boue.