from a couple weekends ago.

From a Google Search about the name:

America's most-painted building received its name in an impulsive exclamation by Lester Hornby. This illustrator and etcher taught in Paris in the winter; his pupils, in the French manner, drew certain standard subjects or motifs.
During his summer seasons in Rockport, Hornby noted that many pupils chose the venerable, dilapidated shed on the edge of the inner harbor. Its prominence and its simple but interesting proportions made it a natural model for sketches and paintings, good and bad. One day when a student brought for criticism a pencil drawing of the house, Hornby exclaimed, "What-Motif No 1 again!" It has been that ever since."
- Rockport Sketch Book, by John L. Cooley