WordNet sense 1 (lacking in courage and manly strength and resolution; contemptibly fearful):

pusillanimous, unmanly

WordNet sense 2 (lacking conviction or boldness or courage):

timid, faint-hearted

WordNet sense 3 (treacherously cowardly):

dastard, dastardly

WordNet sense 4 (lacking even the rudiments of courage; abjectly fearful):

craven, recreant

WordNet sense 5 (easily frightened):

yellow, white-livered

WordNet sense 6 (lacking courage; ignobly timid and faint-hearted):

fearful