The Collaborative International Dictionary of English v.0.48:

Sprack \Sprack\ (spr[a^]k), a. [Cf. Icel. spraekr sprightly,
   dial. Sw. spr[aum]k, spr[aum]g, spirited, mettlesome; or
   Gael. spraic vigor.]
   Quick; lively; alert. [Prov. Eng. & Scot.]
   [1913 Webster]