I'm looking for a word that means saying something with an insincerely serious tone, usually for comedic effect. I feel like the word facetious is close to what I'm looking for, but not exactly.
"Surely, you can't be serious," they said. "Of course I'm serious," he replied [word-ly], "And don't call me Shirley."
The word deadpan is probably the best fit. It can be used as an adjective or adverb (or a noun or a verb):
adjective
marked by or accomplished with a careful pretense of seriousness or calm detachment; impassive or expressionless:
deadpan humor.adverb
in a deadpan manner:
He spoke his lines utterly deadpan.Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2018.
straight faced TFD straight′-faced′ (-fāst′) adj. straight′-faced′ly (-fāst′lē, -fā′sĭd-lē) adv.
A face that betrays no sign of emotion.
As in:
He was a very straight-faced prof, but he has a tremendous sense of humor.
She was watching me totally straight-faced and it took a couple of seconds before it dawned on me that I'd just been had.
And here is an article on some famous straight-faced comedians: article
The word that came to my mind was "dry wit". A search of "dry wit" will bring you to Laurel's answer. Laurel's answer is better anyway, and more common.
dry
14. Humorous in an understated or unemotional way: dry wit.
definition of dry
sarcastic
Sarcasm is an ironic or satirical remark that seems to be praising someone or something but is really taunting or cutting. Sarcasm can be used to hurt or offend or can be used for comic affect.
sardonic
Sardonic comes from the Greek adjective Sardonios, which actually describes a plant from the island of Sardinia that supposedly made your face contort into a horrible grin...right before you died from its poison. The Greeks used sardonic for laughter, but we only use it when someone's humor is also mocking or ironic.
This is a form of understated humor. As others have suggested "deadpan" and "dry wit" are typical forms of understated humor. There is another word that describes it, and that is "Laconic"
A deadpan reply is usually also laconic, delivered tersely, without too many words and faking disinterest.
(...) “Don't look at me,” he said in a conversational tone. “I am slipping a little Browning into the pocket of your oilskin. There — do you feel?” “It is very heavy,” she said. “What am I to do with it?” “Shoot,” he said laconically (...)
What about the word whimsical (made or done for fun, not seriously):
"Surely, you can't be serious," they said. "Of course I'm serious," he replied whimsically, "And don't call me Shirley."
Another good one would be glibly (speaking or spoken in a confident way, but without careful thought or honesty):
"Surely, you can't be serious," they said. "Of course I'm serious," he replied glibly, "And don't call me Shirley."
Mock-sententiously is an expression commonly used in talking about this sort of thing, if the type of serious air in question is self-important.
You might find this discussion of satire helpful, as it defines the use of mock-heroic in parody.
For this context, I would consider slyly. Even if it might not be exactly what are you asking for, it still might be the word you are looking for. From dictionary.com:
in a roguish or mischievous way:
Some were serious, while others slyly winked at the camera.
"Surely, you can't be serious," they said. "Of course I'm serious," he replied slyly, "And don't call me Shirley."
verb (used without object)
to speak or act in mere sport, rather than in earnest; trifle (often followed by with):
Please don't jest with me.
(of course this "thing said or done" can vary according to many criteria, but is still applicable across this "comedic spectrum").
