Source : Webster's Revised Unabridged Dictionary (1913)

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