Flint knife or spearhead, often referred to as a Peseshkef knife by Egyptologists. Ripple flaked with finely serrated forked end resembling a fishtail. The pointed end would have been hafted to a handle. An article by Ann Macy Roth in 'The Journal of Egyptian Archaeology' vol. 78 discusses the possible ritual nature of such a tool.
Marked 3402 (seemingly William MacGregor's collection number).