The Lower layer command lowers the active
layer one position in the layer stack. If the active layer is already at
the bottom of the stack or if there is only one layer, this menu entry
is insensitive and grayed out.
7.16.1. Activating the Command
You can access this command from the image menubar through
Layer → Stack → Lower Layer,
or by clicking on the down-arrow icon at the bottom of the Layers