studly (adj.)

[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]

suffix forming adjectives from nouns and meaning "having qualities of, of the form or nature of" (manly, lordly), "appropriate to, fitting, suited to" (bodily, earthly, daily); irregularly descended from Old English -lic, from Proto-Germanic *-liko- (Old Frisian -lik, Dutch -lijk, Old High German -lih, German -lich, Old Norse -ligr), related to *likom- "appearance, form" (Old English lich "corpse, body;" see lich, which is a cognate; see also like (adj.), with which it is identical).