The Collaborative International Dictionary of English v.0.48:

Semiproof \Sem"i*proof`\ (s[e^]m"[i^]*pr[=oo]f`), n.
   Half proof; evidence from the testimony of a single witness.
   [Obs.] --Bailey.
   [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

SEMI-PROOF, civ. law. Presumptions of fact are so called. This degree of
proof is thus deaned[?]: "Non est ignorandum, probationem semiplenam eam
esse, per quam rei gestae fides aliqua fit judici; non tamen tanta ut jure
debeat in pronuncianda sententia eam sequi." Mascardus, De Prob. vol. 1,
Quaest. 11, n. 1, 4.