Air carrier ready for departure at DAL misinterprets ATC clearance; 'unable position and hold' and taxied into position.

Narrative

Taxiing to Runway 13R in Dallas just short of the hold short line; I was braking the aircraft to a stop. A Company aircraft was just crossing the threshold landing on 13R. Tower evidently said 'unable position and hold'. I did not hear the unable in unable position and hold. I was expecting a position and hold clearance. The First Officer did not understand the clearance either. I assumed I had a position and hold clearance and taxied across the hold short line. The Tower then said 'unable position and hold' which I understood and stopped the aircraft just on the other side of the hold short line. I was not close to the runway as I was still facing northwest on Taxiway C. The Tower said; 'I see you crossed the hold short line; go ahead and position and hold'. I did and when we were cleared for takeoff; we departed 13R without incident. No conflict with any other aircraft during the crossing of the hold short line. I should have made sure I understood the clearance before moving the aircraft. Although the phrase unable position and hold lends itself to misunderstanding; I still should have verified the clearance.