stud (n.1)

[post; also ornamental knob] Middle English stode, from Old English studu "pillar, prop, post, upright timber used as a support," from Proto-Germanic *stud- (source also of Old Norse stoð "staff, stick," properly "stay," Middle High German stud, Old English stow "place"), from PIE root *sta- "to stand, make or be firm."

In later use especially "one of the small, single-story beams of a building which form a basis for the walls."

The word in the sense of "nail-head, knob" used as a fastener, button, etc. is attested by late 13c. The common notion is "fixed in and projecting from a surface."

This sense expanded by early 15c. to include ornamental bosses or devices fixed in and projecting from a garment.

[horse used for breeding] Middle English stode "a herd of horses; place where horses are kept" for breeding or any purpose, from Old English stod, from Proto-Germanic *stodo (source also of Old Norse stoð, Middle Low German stod, Old High German stuot "herd of horses," German Stute "mare").

This is reconstructed to be from PIE root *sta- "to stand, make or be firm," with derivatives meaning "place or thing that is standing" (source also of Old Church Slavonic stado "herd," Lithuanian stodas "a drove of horses").

The shift of sense to "male horse kept for breeding" is attested by 1803, originally colloquial, perhaps via or encouraged by the use of stud-book "publication giving pedigrees of current racing horses" (by 1793). The further-extended meaning "man who is highly active and proficient sexually" is attested by 1895, also extended to dogs. The meaning "any young man" is from 1929.

Studdery "place for keeping a stud of horses" is from 1580s. Stud-poker (1864) is said to be from stud-horse poker, but that phrase seems to be not found earlier than 1876.

Reno is a moral town. Our last faro bank has been compelled to close out business. The boys now play stud horse poker and pedro for amusement. [Weekly State Journal, Reno, Nevada, March 11, 1876]

early 15c., "set with or supply (a wall) with upright timbers;" from stud (n.1) in its older sense. From c. 1500 (implied in studded) as "furnish with metal bosses or nail-heads." In general use, as in studded with "as though sprinkled with" small conspicuous or protuberant objects of any kind, by 1790.