The Collaborative International Dictionary of English v.0.48:

Estop \Es*top"\, v. t. [imp. & p. p. Estophed; p. pr. & vb. n.
   Estopping.] [OF. estoper to stop, plug, close, F.
   ['e]touper, LL. stuppare to close with tow, obstruct, fr. L.
   stuppa tow, oakum, cf. Gr. sty`pph. Cf. Stop.] (Law)
   To impede or bar by estoppel.
   [1913 Webster]

         A party will be estopped by his admissions, where his
         intent is to influence another, or derive an advantage
         to himself.                              --Abbott.
   [1913 Webster]