A fanterm used to describe the yaoi (boyxboy) pairing of Francis (or Francois in some cases) Bonnefoy and Alfred F Jones, from the anime and manga series Axis Powers Hetalia. They are also called or known as the Food Lovers pair. Not as popular as some of the other fanservices, but still enough to have a multitude of fanfics and fanart. The two names are the human names of the humanized countries they personify, Francis being France and Alfred being America (USA).
The "Fr" part stands for France and the "US" for America.
Most of the time, when the pairing is seen, France is the seme, or dominant one in the relationship while America is the uke, the submissive one. But, in rare occasions, the roles can be switched and France would be the uke under America. Though it is not found as much, either way are popular among the many fans that support the pairing.
I love that FrUS fanfic
you showed me the other day!
Those FrUS pics were so cute!!
FrUS is my favorite pairing in Hetalia