Source : Webster's Revised Unabridged Dictionary (1913)

Unfirm \Un*firm"\, a.
   Infirm. [R.] --Dryden.

Source : WordNet®

unfirm
     adj 1: not firmly or solidly positioned; "climbing carefully up the
            unsteady ladder"; "an unfirm stance" [syn: {unsteady}]
     2: (of soil) unstable; "shifting sands"; "unfirm earth" [syn: {shifting}]