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.

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.  

Lock
Slang for a "sure" winner.  

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