The Collaborative International Dictionary of English v.0.48:

Unplumb \Unplumb\
   See plumb.

The Collaborative International Dictionary of English v.0.48:

Unplumb \Un*plumb"\, v. t. [1st pref. un- + plumb.]
   To deprive of lead, as of a leaden coffin. [R.] --Burke.
   [1913 Webster]