What does the word propriety mean in the context?

"Though the law of England, on the subject of the press, is as servile to this day as it was in the time of the Tudors, there is little danger of its being actually put in force against political discussion, except during some temporary panic, when fear of insurrection drives ministers and judges from their propriety;"(source: On Liberty By John Stuart Mill)
What does the underlined word propriety mean? I did look it up but could not figure out the meaning for the context. For me, neither being appropriate or conformity sounds right.

Could you help me clarify it? Thank you always.