B737 DEPARTED FROM MDW BEGINS TKOF ROLL WITHOUT CLRNC. ATC ABORTS TKOF.

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IT WAS THE FIRST LEG OF THE PAIRING IN THE MORNING. AFTER PUSHBACK FROM B18; WE WERE GIVEN CLRNC TO TAXI TO RWY 31C. AS WE APCHED RWY 31R; I SWITCHED TO TWR. THE CAPT ASKED IF WE WERE CLRED TO CROSS RWY 31R; SO I QUERIED TWR. HE SAID GND CTL HAD CLRED US; AND THEN WE THOUGHT HE GAVE US CLRNC TO TAKE OFF ON RWY 31C. WE TAXIED ACROSS RWY 31R TO RWY 31C; LINED UP AND STARTED THE TKOF ROLL. A FEW SECONDS LATER; I THOUGHT TWR MAY HAVE SAID; 'COMPANY FLT NUMBER; STOP.' BUT IT WAS A LITTLE HARD TO HEAR OVER THE ENG NOISE; AND HE DIDN'T SOUND ALARMED. I QUERIED HIM AND HE REPLIED; 'COMPANY FLT NUMBER; ABORT THE TKOF AND HOLD SHORT OF RWY 22L.' WE COMPLIED WITH THAT. HE THEN TOLD US WE COULD TURN R; L; OR DO A 180 DEG TURN. WE SAID WE'D TURN R AND HE CLRED US BACK ONTO HOLD RWY 31C. WE ASKED IF HE HAD GIVEN US TKOF CLRNC AND HE SAID HE DID NOT. WE BOTH THOUGHT HE DID. AFTER ARR INTO SAN; WE CALLED MDW TWR. THEY SAID THAT THERE WAS NO CONFLICT OR INCURSION AND DIDN'T SEEM TOO UPSET. THE CTLR SAID THAT HE DID REVIEW THE TAPES AND CONFIRMED HE DID NOT GIVE US TKOF CLRNC. ALTHOUGH; HE SAID HE DID GIVE US SOME OF THE TKOF INFO ON THE 'ON TO HOLD' CLRNC AND HE COULD SEE HOW WE MADE THE MISTAKE. IT WAS AN ABNORMALLY QUIET DAY AT MDW WITH NO TFC LNDG OR TAKING OFF ON RWY 31C BEFORE US THAT WE WOULD HAVE TO WAIT ON. WHEN HE CLRED US ON TO HOLD; HE GAVE US SOME OF THE INFO THAT THEY USUALLY GIVE WITH TKOF CLRNC. AFTER WE DID THE REST OF THE CHKLIST; IT JUST FELT LIKE WE HAD TKOF CLRNC. I USUALLY MAKE A POINT OF REPEATING THE CLRNC AFTER THE CHKLIST IS COMPLETE; BUT I GUESS THAT HABIT PATTERN FAILED ME ON THE FIRST LEG OF THE PAIRING. SUPPLEMENTAL INFO FROM ACN 600699: I THINK THAT THE UNUSUAL NATURE OF THE 'POS AND HOLD' CALL; COMBINED WITH THE VERY HIGH LEVEL OF COMFORT WITH HOME BASE MDW LED TO AN INITIAL LACK OF COMPLETE FOCUS ON THE JOB AT HAND.