B737 pilot reported the Captain's sliding window opened during takeoff roll resulting in rejected takeoff.

Narrative

During a standard takeoff from Runway XXC at ZZZ; an incident occurred shortly after initiating the takeoff roll. With power set and airspeed between 60 and 70 knots; the captain's sliding window unexpectedly opened fully to its maximum position. Rather than immediately rejecting the takeoff; the captain attempted to close the window as an initial corrective action. During this attempt; the window dislodged completely from its sliding tracks and struck the captain's shoulder. At this point; with airspeed at 109 knots; the decision was made to reject the takeoff; and we executed the Rejected Takeoff (RTO) procedures in accordance with the Aircraft Operating Manual (AOM).Following the RTO; we coordinated with the tower; ground control; and ramp personnel to reposition the aircraft to the west hard stand for brake cooling and monitoring. We completed the AOM RTO checklist during this time. As a precautionary measure; we requested the fire department to assess brake temperatures for safety verification. After approximately 15-20 minutes; once peak brake temperatures had been reached and subsequently began to stabilize; we taxied back to the gate without further incident.Unexpected Window Malfunction: The captain's sliding window opened inadvertently during the takeoff roll; likely due to a mechanical failure or inadequate preflight inspection of the window's locking mechanism. Delayed Decision to Reject Takeoff: The captain's initial attempt to close the window instead of immediately rejecting the takeoff allowed airspeed to increase to 109 knots; complicating the RTO and increasing risk. This falls on both of our parts. I should have called 'reject' sooner. He was distracted with the window. Window Structural Failure: The window dislodged from its tracks during the attempted closure; indicating a possible design flaw; wear; or maintenance oversight; which then physically impacted the captain.Risk Mitigation Strategies: Enhanced Preflight Checks: Implement a more thorough inspection of the cockpit sliding windows during preflight procedures to ensure proper function and secure locking; including a physical test of the mechanism. Full open and full closed. Revised Decision-Making Protocol: Reinforce training and standard operating procedures (SOPs) to prioritize an immediate RTO in the event of a significant anomaly (e.g.; window opening) during critical phases of flight; rather than attempting in-flight corrections below V1. Maintenance Review: Conduct a detailed inspection and maintenance review of all sliding window assemblies across the fleet to identify and address potential wear; design vulnerabilities; or installation issues.