Adam Clarke on Job 20:12

Though wickedness be sweet in his mouth - This seems to refer to the secret sins mentioned above.

Hide it under his tongue - This and the four following verses contain an allegory; and the reference is to a man who, instead of taking wholesome food, takes what is poisonous, and is so delighted with it because it is sweet, that he rolls it under his tongue, and will scarcely let it down into his stomach, he is so delighted with the taste; "he spares it, and forsakes it not, but keeps it still within his mouth," Job 20:13. "But when he swallows it, it is turned to the gall of asps within him," Job 20:14, which shall corrode and torture his bowels.