The term "feist" - an old Southern word - means a small, yappy, bouncy dog of indeterminate parentage.
I will say that my Labs are not so mellow as all that, being field bred.
My 100% field bred black Lab is so feisty, she rushed my camera at age 7 weeks:
'course, she just wanted to give me a great big sloppy puppy kiss, not eat me alive. After all, I had just rescued her from the cargo hold of a jetliner and an open baggage cart on the apron at Hartsfield Atlanta.