[HHKrepublican_2]

could someone please explain to me what a fifth columnist is?

[Leroy S. Mort]

http://en.wikipedia.org/wiki/Fifth_column

[Canedawg]

fifth column n. A clandestine subversive organization working within a country to further an invading enemy's military and political aims.

[posterchild]

Fifth columnist defined here - http://ask.yahoo.com/20000110.html