What is a dry electron?
A "dry" electron is an electron released from the event of an ionization. Depend on source the electron can either be defined as:


  "The $e^-$ is a 'dry electron' that still retains some of the excess
  kinetic energy acquired from the ionizing event."$\ce{^{[1]}}$


or,


  "The 'dry-electron' (defined as an electron before solvation)
  ..."$\ce{^{[2]}}$


A dry electron is thus, an electron that is not assiciated with any atoms, molecules or solutions.

Reference

$\ce{[1]}$ Thormod Henriksen, David H. Maillie; Radiation and Health; page 157

$\ce{[2]}$ J.D. Simon; Ultrafast Dynamics of Chemical Systems; page 144