Eunuchs are at risk of developing osteoporosis so they must take Hormone Replacement Therapy, which artificially replaces testosterone lost by removing the testicles.
It is of course common to refer to a man with the title "Mr." (short for mister), and a woman with the titles "Ms.", "Mrs.", or "Miss". The title "Eu." is used when referring to a Eunuch.
"Ten years ago we called him 'Mr. Jones', but since the operation, we call him 'Eu. Jones'."
There is no known historical basis to this term.