The Collaborative International Dictionary of English v.0.48:

Purpresture \Pur*pres"ture\, n. [Probably corrupted (see
   Prest) fr. OF. pourprisure, fr. pourprendre: cf. LL.
   purprestura. Cf. Purprise.] (Law)
   Wrongful encroachment upon another's property; esp., any
   encroachment upon, or inclosure of, that which should be
   common or public, as highways, rivers, harbors, forts, etc.
   [Written also pourpresture.]
   [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

PURPRESTURE. According to Lord Coke, purpresture, is a close or enclosure,
that is, when one encroaches or makes several to himself that which ought to
be in common to many; as if an individual were to build between high and low
water-mark on the side of a public river. In England this is a nuisance; and
in cases of this kind an injunction will be granted, on ex parte affidavits,
to restrain such a purpresture and nuisance. 2 Bouv. Inst. n, 2382; 4 Id. n.
3798; 2 Inst. 28; and see Skene, verbo Pourpresture; Glanville, lib. 9, ch.
11, p. 239, note Spelm. Gloss. Purpresture Hale, de Port. Mar.; Harg. Law
Tracts, 84; 2 Anstr. 606; Cal. on Sew. 174 Redes. Tr. 117.