An inexperienced or imcompetent police officer, more likely to be found on a small town police force or rural sheriff's department than in a big city. Derived from a character played by Don Knotts on the old Andy Griffith show.
That hayseed deputy who pulled me over last week was a real barney fife.
To put and end to a task or thought in it's early stages.
To be used in place of the phrase "Nip it in the bud", as Barney Fife (Don Knotts) from The Andy Griffith Show used to say.
Adam: "I heard Joe wants to ask your sister out to the club Saturday night."
Kevin: "That scumbag? I'd better Barney Fife that idea right away!"