A BE55 PLT MISHEARD ATC INSTRUCTION AND DEPARTED RSW WITHOUT CLRNC.

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DEPARTING RSW AT NIGHT AFTER RUNUP; I CONTACTED THE TWR AND WAS GIVEN THE FOLLOWING CLRNC: 'TAXI INTO POS AND HOLD. TFC IS A HELI XING THE W END FROM THE S.' I MISTAKENLY HEARD: 'CLRED FOR IMMEDIATE. TFC IS A HELI XING....' I SCANNED FOR AND LOCATED THE TFC AND REPLIED 'GOT THE TFC; ROLLING.' JUST AFTER ROTATING AND LIFTING OFF; THE CTLR CLRED ME FOR TKOF AND ASSIGNED A HDG (RWY). AT THIS POINT THE HELI HAD JUST CROSSED THE FAR END OF THE RWY. I MUMBLED AN APOLOGY TO THE CTLR AND READ BACK HIS CLRNC INCLUDING THE HDG. SINCE THE RWY IS 12000 FT LONG AND I HAD CHOSEN TO USE THE WHOLE THING FOR TKOF; I WAS STILL ABOUT 1 MI FROM THE HELI WHEN IT CROSSED THE RWY. I HAVE AN ONBOARD CLRNC RECORDER SO I PLAYED BACK THE FIRST TKOF CLRNC AND MY REPLY. THE CTLR CLRLY SAID 'POS AND HOLD' SO I CAN ONLY GUESS THAT MY MIND HEARD 'TKOF' ONLY BECAUSE THAT'S WHAT I EXPECTED. I DO BELIEVE THAT THE MENTION OF THE 'TFC XING' CREATED A DISTR WHICH CONTRIBUTED TO MY MENTAL LAPSE. I SUSPECT THAT THE CTLR NEVER REALIZED MY MISTAKE OR ELSE HE DETERMINED THAT IT DIDN'T RESULT IN A DANGEROUS SIT AS THERE WAS NO MENTION OF MY TRANSGRESSION ON THE AIR. THE FIELD WAS VERY QUIET AT THE TIME. OTHER FACTORS: THIS WAS THE FIRST TIME I WAS AT RSW. I MAY HAVE FELT 'RUSHED' TRYING TO KEEP OUT OF THE WAY OF THE AIRLINER TFC. WHEN I LANDED EARLIER THE SAME DAY I WAS ASKED TO TAXI 'SMARTLY' TO SPD THINGS UP AND I COMPLIED HAPPILY (AND WAS THANKED). ALSO THE TRIP WAS TO AN ARPT ONLY 20 MINS AWAY AND I HAVE NOTICED THAT I TEND TO USE LESS CAUTION ON SHORT TRIPS; PROBABLY DUE TO A SUBCONSCIOUS THOUGHT THAT NOTHING BAD COULD HAPPEN ON SUCH A SHORT TRIP. I WAS THE SOLE OCCUPANT OF THE ACFT.