The Collaborative International Dictionary of English v.0.48:

Footstool \Foot"stool`\, n.
   A low stool to support the feet of one when sitting.
   [1913 Webster]

WordNet (r) 3.0 (2006):

footstool
    n 1: a low seat or a stool to rest the feet of a seated person
         [syn: footstool, footrest, ottoman, tuffet]

Easton's 1897 Bible Dictionary:

Footstool
   connected with a throne (2 Chr. 9:18). Jehovah symbolically
   dwelt in the holy place between the cherubim above the ark of
   the covenant. The ark was his footstool (1 Chr. 28:2; Ps. 99:5;
   132:7). And as heaven is God's throne, so the earth is his
   footstool (Ps. 110:1; Isa. 66:1; Matt. 5:35).