點解'tongue-in-cheek' 解作不可當真

tongue-in-cheek' is an adj , suggesting ironic, shyly, humorous. 所以中文譯作 [不可當真], [不可當真],

'tongue-in-cheek' is an adv, suggesting one' tongue in one's cheek above, it is impossible.

According to my Oxford Advanced Learner's English-Chinese Dictionary, with (one's) tongue in (one's) cheek means not intedning to be taken seriously; with irony or humor; and mean it as a joke. For example, Don't be fooled by all his complimentary remarks - they were all said with tongue in cheek.
a tongue-in-cheek remark = 言不由衷的話