An Etymological Dictionary of Astronomy and Astrophysics

1. To assume (something), as for the sake of argument or as part of a proposition or theory.
   

2. To consider (something) as a possibility suggested or an idea or plan proposed (Dictionary.com).

Etymology (EN): M.E. supposen, from O.Fr. supposer, from L. supponere “to put or place under,” from → sub- “under” + ponere “to put, place,”
→ position.

Etymology (PE): Engâštan, engâridan “to suppose,” from Mid.Pers. (h)angârtan “to conside, to bear in mind, to regard as,” from han, ham “together” → com- + kartan “to establish; to declare; to found;” Av. han-kârayeiti, from han-, ham- “together,” + kar- “to remember; to impress on memory.”