I was having lunch in the city on Friday with a couple other writers and
editors, and the subject of slush came up.  Eventually, we got around to
discussing how the term "slush" came about.
 
No one knew for sure where the term came from, but Gordon recalled something
he’d heard once, that if not true, at least makes sense.  The gist of it is:
 
The term slush came about back in the old days, when writers would actually hand
deliver manuscripts to the magazine’s offices.  When the offices weren’t open,
the writers would toss their manuscripts over the transom (what’s
a transom?), into the office.  So when the editors came to work (or back
from lunch), they would have to wade through the pile of submissions, which was
in some way similar, apparently, to wading through slushy snow.  Thus, the slush
pile.  This is also appears to be the source of the term "over the transom."