A hairpin is used by women to hold things in there hair. It is usually made of metal and has the shape of a "U" with long sides. So the expression describes a turn where the vehicle coming out of the turn is going in nearly the opposite direction than it was in coming into the turn. Hope that helps.

thank you:-)

http://thumbs.dreamstime.com/thumblarge_157/11822680406X31eG.jpg

A very sharp turn/bend in the road. Requires slow speed and caution. Usually found in mountainous regions.