stud 2 
(stŭd)
(stŭd)n.
Idiom: 1.
a. A group of animals, especially horses, kept for breeding.
b. A male animal, such as a stallion, that is kept for breeding.
c. A stable or farm where these animals are kept.
2. Slang
a. A usually young man who is very sexually active or promiscuous.
b. A usually young man regarded as attractive and physically fit.
3. Slang A person who is extremely competent in a given area.
4. Games Stud poker.
at stud
Available or offered for breeding. Used of animals.
[Middle English stod, establishment for breeding horses, from Old English stōd; see stā- in the Appendix of Indo-European roots.]