без доп. lose heart, be cast down, be dejected/depressed;
despond не унывай! ≈ cheer up!, don't give up!
уныв|ать - несов. lose* heart, feel*/be* glum, be* depressed/dejected;
не ~ай! cheer up!, don`t give up!
не ~ай! cheer up!, don`t give up!
Большой англо-русский и русско-английский словарь. 2001.