In Greek mythology,
Rhadamanthus, son of
Zeus and
Europa, was much renowned for his sense of strict justice. According to Plato's writings, after Rhadamanthus died, he was sent to Elysium, where the most-favored
demigods and mortals were chosen by the gods to stay eternally, and there he became ruler and judge. Together with
Minos and
Aeacus, he decided the fate of every soul brought before him: whether to allow this soul to live forever in the paradise of
Elysium, or banish him to the not-quite-so-pleasant
underworld.
In this process of judging, Rhadamanthus was able to detect all sins committed by a mortal, no matter how well-concealed. Rhadamanthus's name became associated with justice in its most honest, severe and rigorous form.
Hence, "rhadamanthine" has come to mean strictly and uncompromisingly just.
The term usually prefaces the word "judgment" or "warning."
Sources:
http://www.quinion.com/words/weirdwords/ww-rha2.htm
http://www.bartleby.com/61/51/R0215100.html