Why Hfr cells are called high frequency recombination cells?
According to Luca Cavalli-Sforza, the scientist who coined the acronym Hfr:


  Recombination as observed at the beginning had a very low frequency. It stopped being rare when I found a mutant strain which I called Hfr for “high frequency of recombination.” I found it accidentally in 1949 while I was selecting mutations resistant to nitrogen mustard and radiation. The first two resistant mutants, which had undergone a rather heavy treatment in the process of selecting for resistance to nitrogen mustard, proved to be exceptional in their mating behavior. One was Hfr and it showed immediately its remarkable mating ability, which was higher than that of normal crosses by a factor of 1000 or more. I repeated the experiment two more times before believing it. The other mutation, as I later proved, was an F- (self-sterile) mutation of an F+
  (fertile) strain. (emphases mine)


Source: Forty Years Ago in GENETICS: The Unorthodox Mating Behavior of Bacteria