And it is always used by people that have lost every other arguement on whatever conflict they are discussing. It is used as a last ditch effort to discount the opinions of those that they disagree with.
The premise put forth by the persons using the chickenhawk slur is that people that have not served in combat are not allowed to argue in favor of combat. Taking that to it's logical end, we could no longer elect a president that had not served in combat lest we not be able to defend ourselves if attacked.
I think it is more appropriate to use that term when describing those who are quick to advocate the use of military force, but intentionally tried to avoid military service themselves. Ergo, it is ok for others to risk their lives just as long as I don't have to. Kerry, Murtha, Zinni, et. al. have used it against Cheney, Perle, and others who managed to avoid service in Vietnam. We have used "draft dodger" against Clinton.
I agree it is meant to silence or demean those who support our military intervention in Iraq.