Neither can I find a definition.
In reading extensively on McCarthyism once upon a bleak time ago, he and Roy Cohn used "fellow traveler" to indicate someone that was sympathetic to communists if not actually a member of the c.party. And then they used tht to claim that aiding and abetting was just as bad, and ruined a couple extra lives/careers thatway. "Pinko" was similar - you weren't quite Red, but you were headed that way.
Here's a more recent use of the terms to give some context:
"Fascinating stuff, the letter argued that Erap's extensive links with high profile Filipino-Chinese businessmen prove beyond the shadow of a doubt that he's a commie pinko.
Jeez, I thought fellow traveler
accusations went out with Whittaker Chambers and the John Birch society, or at least with the fall of the Berlin Wall. "
Altoid - curiously strong.