A problematic situation that will gradually increase in severity until it reaches calamitous proportions, such that the people involved or affected by it will not notice the danger until it is too late to act. It is a metaphor taken from an anecdotal parable about boiling a frog, in which a frog placed in boiling water will immediately try to save itself, but one placed in cool water that is gradually brought to a boil will not notice the heat until it is boiled to death. Drug addiction is often a boiling frog, as many people don't see their addiction as problematic until it has consumed their lives.