be treasonable

See: rebel
References in periodicals archive ?
I suppose that since public school boys in the Foreign Office and M16 stopped spying for the Russians, there is not much to be treasonable about, but I suspect that some of the same characters are back at their old tricks, except this time they are not spying for Russia, but casting doubt on whether Roy and Our Boys will win the World Cup.
It would be treasonable for Insider to fuel this scurrilous gossip any further so, in the interests of national security, we're adhering to royal protocol on this matter.