The Collaborative International Dictionary of English v.0.48:

Jah \Jah\ (j[aum]), n. [Heb. y[=a]h.]
   Jehovah. --Ps. lxviii. 4.
   [1913 Webster] Jahvist

Easton's 1897 Bible Dictionary:

Jah
   a contraction for Jehovah (Ps. 68:4).


Hitchcock's Bible Names Dictionary (late 1800's):

Jah, the everlasting