Green’s Dictionary of Slang

1. (US) just below the small of the back; thus a euph. for the buttocks.

a.1909

in J. Ware Passing Eng. of the Victorian Era 262/2: The next instant a huge bull charged out of the door, and, catching the hero of Valley Forge abaft the wheelhouse, incontinently slammed him into a big apple tree.

2. (also aft to the wheel-house) crazy.

1902

Ade Girl Proposition 109: She sprang a new Series of Curves on him every Week or two. Sometimes he suspected that she had gone aft to the Wheel-House but he didn’t like to say so.