DDustiNN
MK11 Pocket Guide: Koming Soon to the App Store
I think we have a pretty good idea...
http://www.urbandictionary.com/define.php?term=have a yellow streak down your back
"to be a coward, to be cowardly"
https://answers.yahoo.com/question/index?qid=20070717133706AAAjWqW
It's a generally accepted meaning that's been around for quite a while.