No, it's not a topless bathing beach vacation.

breastsummer (BRES-sum-uhr, BREST-, BRES-e-muhr) noun

A horizontal beam supporting an exterior wall over an opening, as a shop
window. Also called breast beam.

I think they call them "headers" now.

Taking the poetry out of carpentry.

Not really. Just changing standards of beauty, I think!

Images! Why does it have to be over an opening? And why would a shop window have to be over an opening? Why couldn't you just have that shop window out over the sidewalk? This is so confusing when you all start writing about structures. You cannot begin to imagine the things I see in my head. Also, why'summer'? Is there a backsummer, too? Or a crownsummer? Or a solesummer?

This from AHD:
Summer:

1. A heavy horizontal timber that serves as a supporting beam, especially for the floor above. 2. A lintel. 3. A large, heavy stone usually set on the top of a column or pilaster to support an arch or lintel.
Middle English, beam, pack animal, from Anglo-Norman sumer, from Vulgar Latin *saum rius, from Late Latin sagm rius, pertaining to a packsaddle, packhorse, from sagma, packsaddle. See sumpter.

The term 'breastsummer' or 'bressumer' appears to have been used mainly in connection with fortification works; castles and like structures. I guess that the term 'breastsummer' was used for a supporting beam at around breast height. Or could it have been a beam at ground level supporting 'breastwork' (a breast-high defensive wall)?

over an opening

Umm, the shop window is the opening, Dub Dub'. The breast summer is over the opening to provide support for the stuff that's over *that.

Dear dxb: in the definition you gave, "pack animal" reminds
me of "sumpter mule".

here's another def, I'm looking for a picture...
Header-
(a) A beam placed perpendicular to joists and to which joists are nailed inframing for a chimney, stairway, or other opening.
(b) A wood lintel.
(c) The horizontal structural member over an opening (for example over a door or window).

here we go:
http://www.claimrep.com/Pictures/Header2256.jpg

I tried searching for etymology of "abreast", meaning side
by side, facing in same direction. All I found was AHD
definition of "troika" (in news about twenty years ago)
Russian for a carriage drawn by three horses. But the pictures I have seen of troikas showed middle horse slightly ahead of the other two.

Ha!!!!!!!! Et'! That picture is worth a thousand beams! Here I am imagining shop windows over openings--sorry, Faldage, but I just couldn't help imagining windows over openings--and Et' goes out and fetches a picture--and instead of a pretty little cake shop with a window over some random opening, I see a bunch of joists and beams that look like some carpenter's take on the haunted forest.

However, thank you for the digging and definitions--all! This was great, dxb:

In reply to:

---

'breastsummer' was used for a supporting beam at around breast height

---

...because the breast height really brought the image home.
Also, the word 'lintel' rang true since I do look for lintels when I traipse about the countryside here when I inspect vacant tobacco barns and old cabins.