Wiktionary
n. (context idiomatic English) Something of trivial value, especially of immediate value.
WordNet
n. anything of trivial value; "Esau sold his birthright to Jacob for a mess of pottage"
Wikipedia
A mess of pottage is something immediately attractive but of little value taken foolishly and carelessly in exchange for something more distant and perhaps less tangible but immensely more valuable. The phrase alludes to Esau's sale of his birthright for a meal of lentil stew (" pottage") in and connotes shortsightedness and misplaced priorities.
The mess of pottage motif is a common theme in art, appearing for example in Mattia Bortoloni's Esau selling his birthright (1716) and Mattias Stomer's painting of the same title (ca. 1640).
Usage examples of "mess of pottage".
The old man had gone in, after having sat outside with them to eat a fresh mess of pottage, and to prepare bowls of fig juice.