Dear WW: I finally found "aboideau" in forthright's phrontistery. It means "tide gate".
Venice for instance has spent millions for a tide gate to minimize destructive effects of
extra high tides. In some places in France, they now have gates to hold high tide from
going out, to make it generate electricity. Original meaning was "dike", but somehow
different from usual dikes, couldn't find details.
Today we have Watergates, to flush out rascals.