conjunction
Used to state a negative consequence that will happen if the preceding condition is not met. Often at the beginning of a sentence.
急いでください。さもないと遅れますよ。
Please hurry up. Otherwise, you'll be late.
もっと勉強しなさい。さもないと試験
Study harder. Or else you'll fail the exam.
More formal and literal 'if that is not the case'; さもないと is more colloquial and carries a stronger sense of warning.
Casual contraction of そうでないと; similar in meaning but more informal than さもないと.
Derived from 然も (samo, 'in that way') + 無い (nai, 'not') + と (conditional particle). The exact historical development is uncertain, but it functions as a fixed conjunction meaning 'if not so'.