You are correct. It was impish. I still have the Presidential directive and the codeword print.
Recall, Saturday was the normal day for a weekly test via the teletype system. An actual alert message was issued (I head that the tech pulled the wrong punched tape down and fed that into the machine instead of the test tape) a please cancel the test message was issued. But it was not a test that could be canceled, because it was not a test. Then they had to issue a full cancellation message to cancel an alert.
I too was working that day, at a TV station in Indianapolis. We did stay on the air, I covered the shift at our co-owned radio station while the announcer there tried to track down more information for us.
The primary station for the Indianapolis region activated the EBS (five seconds no carrier, five seconds carrier, five seconds no carrier) then ran silence while awaiting more information to come.
IMPISH:
inclined to do slightly naughty things for fun; mischievous.
“he had an impish look about him”
synonyms: mischievous, naughty, wicked, devilish, rascally, roguish, playful, sportive;