Valiant means brave when the danger is real and the odds are bad. A valiant defence of a bridge; a valiant neighbour going back into a burning house for the dog.
It belongs to knights, soldiers and news reports about rescues, so it sounds a size larger than brave and a good deal older. Its commonest home in everyday English is a valiant effort or a valiant attempt, and that phrase quietly admits the effort failed: the team made a valiant attempt to save the match and lost by ten. The adverb is valiantly, as in she fought on valiantly. The noun for the quality is valour, spelled valor in American English. Save it for facing danger rather than putting up with dullness — a valiant queue is not a thing.
Valiant runs back through Anglo-Norman vaillaunt and Old French vaillant, the present participle of valoir, to be worth. Behind that stands Latin valeō — to have value, to be worth, to be strong — and a Proto-Indo-European root *h₂welh₁-, powerful or strong. A valiant knight was, in the oldest sense, a man who was worth something.
Proto-Indo-European
*h₂welh₁- — powerful, strong; to rule
Latin
valeō — to have value; to be worth; to be strong; to have influence or power
Old French
vailant
Anglo-Norman
vaillaunt
Middle English
vailaunt — having or showing courage or valour, valiant; characterized by valour; powerful, strong; person of valour or strength; excellent, worthy; beneficial, useful; valuable; legally valid, binding
English, today
valiant
Built from
From Middle English vailaunt (“having or showing courage or valour, valiant; characterized by valour; powerful, strong; person of valour or strength; excellent, worthy; beneficial, useful; valuable; legally valid, binding”)vailant, vailaunt, vailiant, vaillant, vaillaunt, vaillaunte, valiant, valiante, valiaunte, valliaunt, valyant, valyante, valyaunt, valyaunte, vaylaunt, waillant, (chiefly Scotland) vailȝeand, from Anglo-Norman vaillaunt, vaylantvailliant, wallant, and Old French vailant, vaillant (“brave, valiant; having value, valuable”)vailand, vailent, from the present participle of valoir (“to have value; to be worth”), from Latin valeō (“to have value; to be worth; to be strong; to have influence or power”), ultimately from Proto-Indo-European h₂welh₁- (“powerful, strong; to rule”).
Etymology from Wiktionary, CC BY-SA