1640s, a legal word, "held through the favor of another," from Latin precarius "obtained by asking or praying," from prex (genitive precis) "entreaty, prayer" (see pray). Notion of "dependent on the will of another" led to extended sense "risky, dangerous, uncertain" (1680s). "No word is more unskillfully used than this with its derivatives. It is used for uncertain in all its senses; but it only means uncertain, as dependent on others ..." [Johnson]. Related: Precariously; precariousness.