Full Idea
Manliness is not a natural human attribute, otherwise women would be just as brave. It is due to pressure from laws, and this pressure has no free will, but is a slave of convention and criticism.
Clarification
'Law' is the Greek word 'nomos', which also translates as 'convention'
Gist of Idea
Being manly and brave is the result of convention, not of human nature
Source
Plutarch (64: Gryllus - on Rationality in Animals [c.85], 988c)
Book Reference
Plutarch: 'Essays', ed/tr. Waterfield,Robin [Penguin 1992], p.389
A Reaction
This is the first glimmerings of seeing gender as a cultural creation, rather than as a fact. Presumably he takes the same view of some of the supposed feminine virtues.