To explain in a patronizing manner, assuming total ignorance on the part of those listening. The mansplainer is often shocked and hurt when their mansplanation is not taken as absolute fact, criticized or even rejected altogether.
Named for a behavior commonly exhibited by male newbies on internet forums frequented primarily by women. Often leads to a flounce. Either sex can be guilty of mansplaining.
The members of the equestrian community were unimpressed when Bob posted an essay that mansplained a solution to a horsy problem they were all well-acquainted with.