"longshot" means something unlikely because it refers to long range naval gun shots being inaccurate and unlikely to hit their target. These long range shots we

"longshot" means something unlikely because it refers to long range naval gun shots being inaccurate and unlikely to hit their target. These long range shots were known for not only missing their targets but missing them by a wide margin. In US English the common idiom "by a longshot" meaning by a wide margin developed based upon this.