in petto

from The Collaborative International Dictionary of English v.0.48

Petto \Pet"to\ (p[e^]t"t[-o]), n. [It., fr. L. pectus.]
   The breast. [Obsolescent]
   [1913 Webster + PJC]

   {In petto}, in the breast; hence, in secrecy; in reserve.
      [1913 Webster]