Someone who is easily manipulated by others, because they substitute the judgment and/or approval of others for their own. The others can be admired friends, strangers or potential mates whose approval the tool seeks. Especially in interpersonal situations, the tool will seek approval from the other, but fail to exercise their own judgment about whether the other person is good or right for them. Alternately, the tool will allow public figures, advertising, or other mass media to replace or form their own opinions on any number of subjects -- most evident in fashion and music choices (often fads or heavily marketed products of suspect quality or style). Somewhat less obvious are tools whose opinions on current events are parroted from sources thought by the tool to be reliably correct -- if you don't follow current events, or didn't come across the original source, you might not realize they hadn't actually given their opinion much thought beyond memorizing the highlights. The recurring theme is that the tool avoids using their own judgment, sometimes even failing to acquire an ersatz opinion; instead just seeking approval. The tool is an open field for anyone who would like to use them for their own purposes. When they choose very poorly, it is obvious to most that the tool has been manipulated and was foolish to have allowed it. Usually singular in actual usage, but sometimes phrases like 'tool shed' or 'hardware store' are used to refer to groups seen as clearly lacking common sense.
a) Did you hear that guy? "Brown is the new black"? What a tool.
b) That tool paid for her fancy dinners all over town for *at least two months* and only kissed her like once!
c) staff in a hotel: "Who rented out the Pacific Room this weekend?" "'Real Estate Riches' seminar, free admission." "Ha, that'll be a tool barn."