The Collaborative International Dictionary of English v.0.48:
Open-mouthed \O"pen-mouthed`\, a. Having the mouth open; gaping; hence, greedy; clamorous. --L'Estrange. [1913 Webster]WordNet (r) 3.0 (2006):
openmouthed adj 1: with eyes or mouth open in surprise [syn: goggle-eyed, openmouthed, popeyed]