1
general::
About to, will, as in I'm going to start planting now, or Do you think it's going to rain? or We thought the train was going to stop here. This phrase is used with a verb (start, rain, stop in the examples) to show the future tense. Occasionally the verb is omitted because it is understood. For example, That wood hasn't dried out yet but it's going to soon, or Will you set the table?? Yes, I'm going to. [1400s] Also see GO TO.
American Heritage Idioms