The Collaborative International Dictionary of English v.0.48:

Palmetto State \Palmetto State\ prop. n.
   South Carolina; -- a nickname alluding to the State Arms,
   which contain a representation of a palmetto tree.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

Palmetto State
    n 1: a state in the Deep South; one of the original 13 colonies
         [syn: South Carolina, Palmetto State, SC]