From The Collaborative International Dictionary of English v.0.48 :
Sea purse \Sea" purse`\ (s[=e]" p[^u]rs`). (Zool.)
The horny egg case of a skate, and of certain sharks.
[1913 Webster]
From WordNet (r) 3.0 (2006) :
sea purse
n 1: the seaward undercurrent created after waves have broken on
the shore [syn: undertow, sea puss, sea-puss, sea
purse, sea-purse, sea-poose]