(a.) Full of spirit; possessing courage; fiery; as, a mettlesome horse\.

active acute aggressive agitable animated beany brisk bulldoggish courageous dauntless dynamic edgy emotional emotionally unstable energetic enterprising enthusiastic eruptive excitable explosive fiery forceful forcible full of pep game gamy go-go gritty gutsy gutty hearty high-mettled high-spirited high-strung highly emotional impetuous incisive inflammable intense irascible irritable keen kinetic lively living lusty nervous nervy peppery peppy pertinacious perturbable plucky prickly red-blooded resolute robust sensitive skittish smacking snappy spanking spirited spiritful spunky startlish strenuous strong take-charge take-over tenacious touchy tough trenchant vibrant vigorous vivacious vivid volcanic zestful zesty zippy