by 1971, American English, from stud (n.2) in the "virile male" sense + -ly (1). Related: Studliness.
Entries linking to studly
stud n.2
"horse used for breeding," Old English stod "herd of horses, place where horses are kept for breeding," from Proto-Germanic *stodo (source also of Old Norse stoð, Middle Low German stod, Old High German stuot "herd of horses," German Stute "mare"), 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").
Sense of "male horse kept for breeding" is first recorded 1803; meaning "man who is highly active and proficient sexually" is attested from 1895; that of "any young man" is from 1929. Stud-poker (1864) is said to be from stud-horse poker, but that phrase is not found earlier than 1879.
-ly 1
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).