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; and, speaking generally, it is not, in constitutional countries, to be apprehended, that the government, whether completely responsible to the people or not, will often attempt to control the expression of opinion, except when in doing so it makes itself the organ of the general intolerance of the public.
(source: On Liberty By John Stuart Mill)
What does the bolded 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?