Apparently, it is also an important Jewish name? http://www.saudades.org/namesfrombrazil.html
(just search down)
Could it be that the surname refers to the people (Jews or Arabs, who were really common in Portugal, hailing from Syria/Palestine=dry places) or to a specific place in Portugal? This is also really confusing, because it makes no sense, especially considering my father is a very, very dark mistico/mulatto, most probably a descendant of slaves from Brazil! How could this be? Many slaves did come from Mali, which is also dry, so it could also be an adaptation of the surname, but I don't think the Brazilian descendants knew the meaning of Siqueira or their direct origins.