"stag in its fourth year," thus not quite full-grown, c. 1400, from stag + -ard.
Entries linking to staggard
stag n.
late 12c., probably from Old English stagga "a stag," from Proto-Germanic *stag-, from PIE root *stegh- "to stick, prick, sting." The Old Norse equivalent was used of male foxes, tomcats, and dragons; and the Germanic root word perhaps originally meant "male animal in its prime."
Adjectival meaning "pertaining to or composed of males only" (as in stag party) is American English slang from 1848. Compare bull-dance, slang for one performed by men only (1845); gander (n.) also was used in the same sense. Stag film "pornographic movie" is attested from 1968. Stag beetle, so called for its" horns," is from 1680s.
-ard
also -art, from Old French -ard, -art, from German -hard, -hart "hardy," forming the second element in many personal names, often used as an intensifier, but in Middle High German and Dutch used as a pejorative element in common nouns, and thus passing into Middle English in bastard, coward, blaffard ("one who stammers"), etc. It thus became a living element in English, as in buzzard, drunkard. The German element is from Proto-Germanic *-hart/*-hard "bold, hardy" (from PIE root *kar- "hard").