Buck in its fourth year

Douglas Harper's Etymology Dictionary Word definitions in Douglas Harper's Etymology Dictionary
"stag in its fourth year," thus not quite full-grown, c.1400, from stag + -ard .

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
Staggard \Stag"gard\ (-g[~e]rd), n. [From Stag .] (Zo["o]l.) The male red deer when four years old.

Wiktionary Word definitions in Wiktionary
n. A male red deer when four years old.