A319
Flight Crew Air Transport Pilot (ATP)
Communication Breakdown; Confusion
ATC Issue All Types; Deviation / Discrepancy - Procedural Clearance; Ground Incursion Runway
Air Traffic Control Issued New Clearance; Air Traffic Control Issued Advisory / Alert
Human Factors; Procedure
Ground cleared us from pushback to Runway 21 via Taxiway Alpha. We were taxiing northeast bound turning around the corner (right hand turn) Taxiway A southeast bound to the hold short line Runway 21 when Tower gave instructions in a weak and mumbled tone in what sounded like '(inaudible word)...; line up and wait Runway 21; after departure turn left direct Donnelly VOR.'  I then read back the clearance 'Line up and wait Runway 21; after departure turn left direct Donnelly VOR.' I was then corrected by Tower; 'UNABLE line up and wait Runway 21. HOLD SHORT Runway 21.' I then responded; 'UNABLE line up and wait Runway 21; HOLD SHORT Runway 21 turn left direct Donnelly.' As I was responding the Captain was slowing and stopping the aircraft close to the hold short line. The nose of aircraft was over the hold short line but not far enough for the nose gear to be over. We stopped. No aircraft was on final approach at the time. We waited about a minute then Tower cleared us on the runway for takeoff. All else was normal.I should have asked Tower to repeat the instructions when the first word was confusing and inaudible. I have never received a hold short clearance like that before; 'UNABLE; Line up and wait;' only 'Hold short Runway X'. It was confusing BUT I DID READ BACK WHAT I RECEIVED. I then was corrected but just a little too late. The Captain perceived the same clearance as I after reviewing the situation. I think the cause was an unstardard clearance from Tower; 'Unable; line up and wait' plus the fact he said it in a weak and mumbled tone.1 Always question if unsure of a clearance.2 ATC [should employ] clear; slow; standard clearances.
Confusion due to odd phrasing of a taxi hold short clearance resulted in a minor runway incursion at GEG.