It seems that suikinkutsu was first called Tosuimon (a drain facility of the lavatory) that was not built in a garden, but was a facility for drainage of a lavatory basin when washing hands.
水琴窟は最初は洞水門（とうすいもん）と呼ばれており庭園に設置されるものではなく手洗い場の排水設備であったらしい。