Boeing claims to have eliminated the risk of combustion, but not ignition.
So, I took the keys out of the ignition and hid them under my passenger seat.
A jet engine that is still functioning is basically a furnace, and the most likely first source of ignition.
1610s, "act of heating to the point of combustion," from French ignition (16c.) or directly from Medieval Latin ignitionem (nominative ignitio), from Latin ignire "set on fire," from ignis "fire" (see igneous). Meaning "means of sparking an internal combustion engine" is from 1881.