ORD LCL CTLR USED INCORRECT PHRASEOLOGY BY INSTRUCTING AN MD80 TO 'TAXI INTO POS AND HOLD; EXPECT AN IMMEDIATE TKOF; TFC ON A 4 MI FINAL.'

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AWAITING TKOF AT RWY 9L AT ORD. JUST AFTER AN MD80 ROLLED ON RWY 9L AND WITH TFC ON FINAL; TWR INSTRUCTED US TO 'POS AND HOLD RWY 9L; EXPECT AN IMMEDIATE TKOF; TFC ON A 4 MI FINAL.' I HEARD THE 'IMMEDIATE TKOF' CALL AND THE FINAL TFC CALL AND LOOKED TO CLR FINAL. FO COMPLETED THE LAST 5 ITEMS ON THE CHKLIST; BUT HEARD THE POS AND HOLD CALL. WE DID NOT CONFIRM WITH EACH OTHER THE TKOF CLRNC. HE FINISHED THE TKOF CHKLIST JUST AS I ADDED PWR TO BEGIN ROLLING. HE NOTED THE PWR AND SAID 'WE ARE NOT CLRED FOR TKOF; JUST POS AND HOLD.' I STOPPED THEN NOTED AN F100 ROLLING FOR TKOF ON RWY 4L AT THAT MOMENT. THEN TWR IMMEDIATELY CLRED US FOR TKOF. THIS COULD HAVE BEEN SERIOUS; AS I NEVER HEARD THE POS WORDS; BUT RATHER CLRLY HEARD THE TWR SAY 'IMMEDIATE TKOF' AND ASSUMED THAT WE WERE CLRED TO GO. ATTA BOY FOR A SHARP FO WHO DID NOT HESITATE TO BE ASSERTIVE AND FIRM. THE TWR PHRASEOLOGY OF 'EXPECT IMMEDIATE TKOF;' WHILE NOT UNUSUAL; DOES NOT NORMALLY INCLUDE THE WORD 'TKOF' IN THE PHASE. ALSO; THE 5 ITEMS LEFT OPEN DISTR US FROM CONFIRMING WITH ONE ANOTHER AS I USUALLY DO. FATIGUE WAS ALSO A FACTOR FOR MYSELF. SUPPLEMENTAL INFO FROM ACN 625878: WHILE WAITING AT THE DEP END OF RWY 9L AT ORD; TWR CLRED A FLT AHEAD OF US FOR TKOF. IMMEDIATELY AFTER; TWR INSTRUCTED US TO TAXI INTO POS AND HOLD RWY 9L; TFC ON 3 1/2 MI FINAL; BE READY FOR AN IMMEDIATE TKOF. THE CAPT APPLIED PWR AND WE FINISHED THE REMAINING ITEMS ON THE MECHANICAL CHKLIST AS WE ROLLED INTO POS. THE AMOUNT OF THRUST USED AND THE ACCELERATION OF THE ACFT INDICATED TO ME THAT THE CAPT THOUGHT WE WERE CLRED FOR AN IMMEDIATE TKOF. AT THIS POINT; I REMINDED THE CAPT THAT WE WERE ONLY CLRED FOR 'POS AND HOLD.' THE CAPT IMMEDIATELY CLOSED THE THROTTLES AND STOPPED THE ACFT. SHORTLY THEREAFTER; TWR CLRED US FOR TKOF. WE MADE A NORMAL AND UNEVENTFUL TKOF AND FLT TO DFW. IN THE DISCUSSION AFTER; THE CAPT HAD SAID THAT HE HAD HEARD 'CLRED FOR IMMEDIATE TKOF' IN CTLR'S INITIAL CONTACT WITH US AND HAD THOUGHT WE WERE CLRED FOR TKOF AND NOT POS AND HOLD. WHILE THERE WERE NO DEVS BY THE TWR CTLR OR FLT CREW; THE DISTR OF FINISHING THE MECHANICAL CHKLIST WHILE TAKING THE RWY; TFC ON SHORT FINAL; AND THE CTLR'S USE OF THE WORD 'TKOF' IN HIS INITIAL COM LED TO A TEMPORARY MISUNDERSTANDING OF OUR ACTUAL CLRNC.