Deus ex machina means "God From the Machine" or "The God Machine" and is a plot device in which a person or thing appears out of the blue to help a character to overcome a seemingly insolvable difficulty.
Deus ex machina is mentioned twice in the film. The first time by Mrs Pomeroy in the calssroom during the discussion about Watership Down. Later in the film Donnie says it to Seth when they are fighting in the road just as Frank's car appears in the distance. This is when Frank runs over and kills Gretchen.