1. boldly courageous; brave; stout-hearted:
a valiant soldier.
2. marked by or showing bravery or valor; heroic:
to make a valiant effort.
3. worthy; excellent.

valiantly (adverb)
valiantness (noun)

Synonyms: bold, courageous, dauntless, doughty, fearless, gallant, greathearted, gutsy, gutty, heroic (also heroical), intrepid, lionhearted, manful, stalwart, stout, stouthearted, undauntable, undaunted, brave

Antonyms: chicken, chickenhearted, chicken-livered, coward, cowardly, craven, dastardly, fainthearted, fearful, gutless, lily-livered, nerveless, poltroon, poor-spirited, pusillanimous, spineless, spiritless, timorous, uncourageous, ungallant, unheroic, weakhearted, yellow bellied

Prefix: over-, un-

Suffix: -ly, -s, -ness