The term refers to when someone accidentally overdoses on medication and dies from it. It is in reference to Heath Ledger.
to Heath (to accidentally overdose)
to pull a Heath (to start to accidentally overdose)
Heathing (in the middle of overdosing)
Yo man, Johnny ate too many xanaxs last night and he pulled a
HeathGuys! Call 911, Jill is
Heathing