Wonk is a slang term with several meanings:
Wonk (colloquial American English) was originally a 1960s slang word applied to an excessively studious person (equivalent to "grind" or "nerd").  The origins of the term are obscure. It has been described as a simple reversal of "know," linked to an obscure Old English word, and attributed to Royal Navy slang for a learned but inexperienced midshipman.
Presumably from the above, a policy wonk is someone knowledgeable about and fascinated by details of government policy and programs.
Wonk is also an American slang term referring to someone who is sight disabled, as in the expression "He has one wonky eye. He's a wonk."
Wonk is used to designate a fork with just a single prong, in the line of twook, threek, and fork
Hank at Insureblog has posted the latest Health Wonk Review. My blog on disenfranchised physicians is in it. If you have never wonked before, you will not regret the experience. I am not sure which kind of wonk it really is, but I am sure it is worth wonking.