The Collaborative International Dictionary of English v.0.48:

Subsign \Sub*sign"\, v. t. [imp. & p. p. Subsigned; p. pr. &
   vb. n. Subsigning.] [L. subsignare; sub under + signare to
   mark: cf. F. soussigner. See Sign.]
   To sign beneath; to subscribe. [R.] --Camden.
   [1913 Webster]