c. 1200, "human; characteristic of human beings," also "possessing virtues proper to a male person" (resoluteness, independence, steadfastness, reliability); from man (n.) + -ly (1). Meaning "masculine, not boyish or womanish, proper to fighting men" is attested from late 14c. Old English had werlic "male, masculine, manly."
Manly, matching womanly, is the word into which have been gathered the highest conceptions of what is noble in man or worthy of his manhood, especially as opposed to which is fawning or underhand. Manful expresses the stanchness, fearlessness, and energy of a man, as opposed to that which is weak, cowardly, or supine. [Century Dictionary, 1895]
word-forming element denoting action, quality, or state, attached to an adjective or past participle to form an abstract noun, from Old English -nes(s), from Proto-Germanic *in-assu- (cognates: Old Saxon -nissi, Middle Dutch -nisse, Dutch -nis, Old High German -nissa, German -nis, Gothic -inassus), from *-in-, originally belonging to the noun stem, + *-assu-, abstract noun suffix, probably from the same root as Latin -tudo (see -tude).
<a href="https://www.etymonline.com/word/manliness">Etymology of manliness by etymonline</a>
Harper, D. (n.d.). Etymology of manliness. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/manliness