1) Having the mouth open 2) Vociferous or clamorous 
• (a.) Having the mouth open; gaping; hence, greedy; clamorous.
O'pen-mouthed` adjective Having the mouth open; gaping; hence, greedy; clamorous. L'Estrange. 
having the mouth open. · gaping, as with surprise or astonishment. · greedy, ravenous, or rapacious. · clamoring at the sight of game or prey, as hounds. · vociferous or clamorous. · having a wide mouth, as a pitcher or jar.