A B737-800 CREW; ON TKOF ROLL AT JFK; EXPERIENCED THE FO'S SIDE WINDOW OPENING; A REJECTED TKOF ENSUED.

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I WAS THE CAPT AND PF ON ACR X FLT ABCD. STARTED OUR TKOF ROLL ON RWY 4L. AT APPROX 60 KTS; THE FO'S COCKPIT WINDOW OPENED. I IMMEDIATELY ABORTED THE TKOF AND NOTIFIED JFK TWR. AS WE TAXIED BACK TO RAMP; COMPANY WAS NOTIFIED. ALSO; LOGBOOK ENTRY WAS MADE. MAINT INSPECTED FO'S COCKPIT WINDOW. MAINT WROTE OFF LOGBOOK DISCREPANCY AS COCKPIT WINDOW WORKING PROPERLY. PRIOR TO THIS INCIDENT; ALL COMPANY PROCS AND CHKLISTS WERE FOLLOWED. ONE ITEM ON THE CHKLIST IS 'DOORS/COCKPIT WINDOWS...CLOSED/LOCKED.' THE RESPONSE IS TO BE MADE BY BOTH PLTS. I ALWAYS ENSURE THE CAPT'S COCKPIT WINDOW IS CLOSED/LOCKED BY DOING A PUSH/PULL MOTION ON THE HANDLE OF THE CAPT'S COCKPIT WINDOW. THE FO'S COCKPIT WINDOW WAS PRIOR TO RESPONDING TO IT BEING 'CLOSED/LOCKED.' HE STATED THAT HE 'PUSHED' THE HANDLE OF THE COCKPIT WINDOW. WHEN MAINT ASKED HIM TO TRY IT AGAIN; AND THEN HE 'PULLED' ON THE HANDLE; THE FO'S COCKPIT WINDOW OPENED. CORRECTIVE ACTIONS: A PUSH/PULL METHOD SHOULD BE USED TO ENSURE THE COCKPIT WINDOW IS CLOSED/LOCKED. IF THE WINDOW OPENS WHEN YOU PULL THE HANDLE; 1) CLOSE THE WINDOW; 2) PUSH THE HANDLE TO LOCK THE WINDOW; AND 3) PULL THE HANDLE TO ENSURE THE COCKPIT WINDOW IS CLOSED/LOCKED.