What is the best way to convert "tongue-in-cheek" into an adverb?
Solution 1:
(Edit: I reworked my answer, since Merriam-Webster was inconsistent between the free and unabridged versions.)
Per OxfordDictionaries.com, the adverb and adjective form of the phrase "tongue in cheek" is tongue-in-cheek (with hyphens):
[as adjective]: her delightful tongue-in-cheek humour
[as adverb]: I think he was talking tongue-in-cheek.