a- (“in, on”) + building (“the act or process of building”)
a-building (not comparable)
- Being built or under construction, as a structure or a vessel.
1890, William Morris, chapter XXVI, in News from Nowhere:
This talk brought us up to the house that was a-building, not a large one, which stood at the end of a beautiful orchard surrounded by an old stone wall.
1922, Eric Rücker Eddison, The Worm Ouroboros, page 141:
"Of these nineteen ships a-building no more than two can take the water before a month be past, and but seven more ere six months' time, push we never so mightily the work."