1754, "having to do with consent," from stem of Latin consensus (see consensus) + -al (1). Meaning "by consent" is attested from 1800. Until modern times used almost exclusively with reference to legal contracts and to the eyes working together reflexively; it was extended to the language of sociology and psychology from 1950s (of social groups, non-arranged marriages, etc.) and in the legal discussions of rape and other sex crimes by 1977.
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