The Collaborative International Dictionary of English v.0.48:

Conjecturally \Con*jec`tur*al"ly\, n.
   That which depends upon guess; guesswork. [R.] --Sir T.
   Browne.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Conjecturally \Con*jec`tur*al*ly\, adv.
   In a conjectural manner; by way of conjecture. --Boyle.
   [1913 Webster]

WordNet (r) 3.0 (2006):

conjecturally
    adv 1: in a manner involving or inclined to conjecture and
           supposition