Taru adjective, Adverb taking the 'to' particle
triumphant; exultant; in high and proud spirits
Yojijukugo (four character compound)see 揚々
ワールドカップで優勝した全日本チームは、意気揚々と帰国してきた。The Japanese World Cup champion team came back home flushed with victory.
Also written 意気揚揚【いきようよう】