In Romeo and Juliet, what does the line "where civil blood makes civil hands unclean" mean?
This line, from the Prologue, refers to the quarrels between ordinary people and how these can lead to violence and bloodshed. The particular quarrel that this play is concerned with is that between two old noble families of Verona, the Montagues and Capulets.
The key aspect of this line is the word 'civil' which has a double meaning. In the first instance it simply refers to the ordinary people, the citizens of a state (as opposed to the government, army, or legal and religious institutions). However, it can also be taken to denote qualities such as courtesy, respect, restraint, rationality and dignity, which ideally should be present in the people of a supposedly civilized and well-ordered state.
The use of the word 'civil' becomes quite ironic in the Prologue as in the course of the play the Montagues and Capulets will be shown to be anything but courteous and dignified in their dealings with each other. They prosecute an ancient feud which consumes all their energies. There is no rational basis for such enmity, nor does it tend to any worthwhile end. Rather, it causes unnecessary suffering and needless deaths like those of the young lovers, Romeo and Juliet, who would rather indulge positive emotions of love instead of hate. Their tragedy is that they are caught up in this ongoing warfare between their respective families. In the end, their deaths do bring about a reconciliation between the two houses, but there is a sense that it is really too late.
In this Prologue, the Montagues and Capulets are also implicitly condemned for degrading their nobility by their quarrelsome and futile actions. They are of ancient lineage and high rank - 'two households, both alike in dignity' - yet they abjectly fail to set a good example to the rest of society.
No Fear Shakespeare translates this line as: "...and citizens stain their hands with the blood of their fellow citizens." This literally means that the townspeople are killing each other. This line sets up the play; it tells the reader that lives will be lost, and there will not be an entirely happy ending.