The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
8. And on account of thoughtfulness.
Moreover also the word 'thoughtfulness' denotes a lengthened carrying on of the same train of ideas. Now 'thoughtfulness' we ascribe to those whose mind is concentrated on one and the same object, while their look is fixed and their limbs move only very slightly. We say e.g. that the crane is thoughtful, or that a wife whose husband has gone on a journey is thoughtful. Now such thoughtfulness is easy for those who sit; and we therefore conclude herefrom also that meditation is the occupation of a sitting person.