The Collaborative International Dictionary of English v.0.48:

Keslop \Kes"lop\ (k[e^]s"l[o^]p), n. [AS. c[=e]selib, or
   c[=y]slyb, milk curdled; cf. G. k[aum]selab, k[aum]selippe.
   See Cheese, and cf.Cheeselep.]
   The stomach of a calf, prepared for rennet. --Halliwell.
   [1913 Webster]