A type of headache caused by cold around the brain, either at the back of the throat when eating icecream (as the name suggests), or from cold around the outside of the head. The latter is well known to surfers in cold windy conditions.
1997: In my wilder days as a winter surfer we all knew about the sickening frontal headache that resulted within seconds of driving through a breaking wave. The pain continued for 20 to 30 seconds, only to be reinforced by the next breaking wave. Even then (35 years ago) we always referred to the pain as the ice cream headache. — physician Mark Harris, letter to the British Medical Journal, 315:609 (6 September 1997)