Source : Webster's Revised Unabridged Dictionary (1913)

Key \Key\, v. t. [imp. & p. p. {Keved}; p. pr. & vb. n.
   {Keying}.]
   To fasten or secure firmly; to fasten or tighten with keys or
   wedges. --Francis.

   {To key up}.
   (a) (Arch.) To raise (the whole ring of an arch) off its
       centering, by driving in the keystone forcibly.
   (b) (Mus.) To raise the pitch of.
   (c) Hence, fig., to produce nervous tension in.