The Collaborative International Dictionary of English v.0.48:

Incertum \In*cer"tum\, a.
   Doubtful; not of definite form.
   [1913 Webster]

   Opus incertum (Anc. Arch.), a kind of masonry employed in
      building walls, in which the stones were not squared nor
      laid in courses; rubblework.
      [1913 Webster]