Abarim Publications' online Biblical Greek Dictionary
The verb μελλω (mello) means to pend, to be impending, to be imminent, to be destined or to be about to.
This verb expresses that something is likely to happen, or that it should happen (in an assumed accordance with propriety, natural or divine order). It may express intent: to be about to do, or impendence: to be about to occur. It may even facetiously express delay: to always be about to do something but never really actually doing it (Acts 22:16).
This fascinating verb occurs mostly in the present tense, and when the narrative requires a past tense, the imperfect is used: "he was being about to." This verb is deployed 111 times in the New Testament, see full concordance, and has no derivations.