Michael Onions has a fine explanation of co-respondent shoes, here:
He mentions in passing that these lounge lizard bits of foot covering are also called ‘spectator shoes’, which the OED says is of U.S. origin. There is a reference to
spectator pumps in the old WordOrigins board, “-- Also called spectator shoe. A white shoe with a perforated wing tip and back trim, traditionally of dark brown, dark blue, or black but sometimes of a lighter color.” http://wordoriginsorg.yuku.com/reply/71899#reply-71899
Does anyone have a thought as to how such things came to be called spectator shoes? From the co-respondent etymology, one is almost tempted (but propriety intervenes?) to think of threesomes!