Definition 1 of 1
(a) they are usually strung together in use and meaning, rather than employed and implied individually
(b) the person using them will often put in a negative verb -- see 2. below -- which, unbeknownst to them, reverses their intended meaning.
Often used by the speaker to suggest strain or hardship upon oneself in the interim (i.e., in the time before the "unless and until" event actually happens).
2. Unless and until that rascal doesn't give my money back, I will make his life hell!