Medieval definitions of masculinity take as their point of departure anatomical considerations, according to which men possess greater physical strength than women. Isidore of Seville's Etymologiae, one of the most authoritative encyclopedias of the Middle Ages, explains that the critical difference between men and women is in their bodily strength. In Book XI, Isidore states that the term vir (man) is a derivative of vis (force), because man is stronger than woman and because he deals with her by force (XI.ii.17); mulier (woman), on the other hand, he claims is derived from mollitia (softness) (18). “These two are differentiated by the respective strength and weakness of their bodies. But strength is greater in a man, lesser in a woman, so that she will submit to the power of the man” (19). The word “respective” is crucial: it indicates the relational aspect of gender, with male strength seen as the measure of all things. “Female” is a mere comparative and exists only in relation to the male; and female inferiority on the physical level is required in order to assert the biological male's masculine qualities. That means that male identity, too, is determined by its relation to the other sex. The “proper” relationship on the level of biology is all the more important because physical superiority is subsequently construed as the basis for moral superiority — when, for example, Isidore explains that virtus (virtue) is derived from vir (X.v.274). Female inferiority on the moral level is another essential bolster to male identity, which is defined by the manifestations of femininity with which it interacts.

Although the Etymologies present physical inequality as the original state of mankind, they nevertheless allow for transgressions of the normative strong-weak dichotomy. Isidore defines the virago as someone who “‘acts like a man’ (vir + agere), that is, she engages in the activities of men and is full of male vigor. […] if a woman does manly deeds, then she is correctly called a heroic maiden, like an Amazon” (XI.ii.22). Rather than undermining Isidore's own account of submissive femininity as standard, this clarifies that strong women are an exception to the general rule. The virago's behavior is described as manly; that confirms that physical attributes (like strength), but also moral virtues (like heroism) are inherently male, even when they are displayed by a woman.