THE COCKPIT SIDE WINDOW CAME OPEN ON TKOF ROLL.

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UPON INITIATION OF TKOF ROLL; COCKPIT NOISE WAS GREATER THAN USUAL. AT APPROX 50-60 KIAS; FO NOTICED THAT CAPT'S SIDE WINDOW WAS NOT LOCKED CLOSED. ATTEMPT WAS MADE TO CLOSE WINDOW. UNABLE TO DO SO; ABORT WAS INITIATED AT APPROX 90 KIAS. WINDOW WAS CLOSED; REQUIRED BRAKE COOL DOWN TIME COMPLETED; AND UNEVENTFUL TKOF AND FLT COMPLETED. SINCE THE CHKLIST CALLS FOR WINDOWS CLOSED IN 2 PLACES; CAUSE OF PROB CAN BE ATTRIBUTED TO LACK OF PROPER CHKLIST DISCIPLINE. EVEN THOUGH THE CHKLIST WAS READ AND ANSWERED PROPERLY; THE CREW MEMBERS BOTH FAILED TO INSURE THAT THE WINDOW WAS IN FACT CLOSED AND LOCKED. SOLUTION WOULD BE TO INSURE THAT EACH ITEM ON THE CHKLIST IS NOT ONLY READ AND ANSWERED PROPERLY BUT THAT THE ITEM IS ACTUALLY ACCOMPLISHED. STRICT CHKLIST DISCIPLINE IS THE KEY. SUPPLEMENTAL INFO FROM ACN 267360: THE AIRPLANE HAD BEEN WITH THE MECHS FOR SEVERAL HRS. I WAS NOT METHODICAL ENOUGH IN MY COCKPIT SETUP AND MISSED THE 'WINDOW LOCKED' HANDLE BEING OUT OF POS; EVEN THOUGH I RECALL LOOKING RIGHT AT IT.