The Collaborative International Dictionary of English v.0.48:

Lopper \Lop"per\, v. i. [imp. & p. p. Loppered; p. pr. & vb.
   n. Loppering.] [Cf. Prov. G. l["u]bbern, levern, OHG.
   giliber[=o]n, G. luppe, lab, rennet.]
   To turn sour and coagulate from too long standing, as milk.
   [1913 Webster]