Lame
A deviation from a normal gait due to pain in a limb or its supporting structures.

Lasix
A medication which acts as a diuretic and aids in breathing  

Late Double
A second daily double offered during the latter part of the program; usually the last two races. 

Lathered Up
A horse that is overly nervous and sweaty is said to be lathered up.  Also referred to as being washed out.

Lead (pronounced led)
Lead weights carried in pockets on both sides of the saddle, used to make up the difference between the actual weight of the jockey and the weight the horse has been assigned to carry during the race.  

Lead (Pronounced leed)
The front leg that is last to hit the ground during a gallop or canter. In order to minimize fatigue, a horse should switch leads at least once during a race. 

Leg Up
1) To help a jockey mount a horse. 2) A jockey having a mount.  

Length
A measurement approximating the length of a horse, used to denote distance between horses in a race. Secretariat won the Belmont by 31 lengths.  

Listed Race
A stakes race just below a group race or graded race in quality.  

Lock
Slang for a "sure" winner.  

Lug (in or out)
Bearing in or (out).