"Frank dehydration" is not really a regular medical term. You can suspect this by using Google search with "frank dehydration" in quotes and site:gov (only 3 websites showing up) and site:edu (2 websites) operators.
The term "frank dehydration" as an informal term can refer to "obvious," "actual," "test-confirmed" dehydration as opposed to feeling dehydrated or thirsty.
Tests to confirm dehydration include a combination of the body exam (dry mouth, prolonged skin turgor, decreased body weight), urine tests (dark urine, increased urine specific gravity, decreased urination frequency and 24-hour urine volume) and blood tests (increased sodium levels - hypernatremia).