The Collaborative International Dictionary of English v.0.48:

Ligan \Li"gan\ (l[imac]"gan), n. [Cf. L. ligare to bind, to tie,
   ligamen band, bandage, E. ligament, or ligsam.] (Law)
   Goods sunk in the sea, with a buoy attached in order that
   they may be found again. See Jetsam and Flotsam. [Written
   also lagan.] --Blackstone.
   [1913 Webster]

WordNet (r) 3.0 (2006):

ligan
    n 1: goods (or wreckage) on the sea bed that is attached to a
         buoy so that it can be recovered [syn: lagan, lagend,
         ligan]

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

LIGAN or LAGAN. Goods cast into the sea tied to a buoy, so that they may be
found again by the owners, are so denominated. When goods are cast into the
sea in storms or shipwrecks, and remain there without coming to land, they
are distinguished by the barbarous names of jetsam, (q.v.) flotsam, (q.v.)
and ligan. 5 Rep. 108; Harg. Tr. 48; 1 Bl. Com. 292.