The Justacorps jacket was a knee-length coat, fitted to the waist and flared below, without a waist seam. This coat possibly brought to Western Europe by Charles II attempted to emulate some of the design cues seen in Eastern robes at the time. In time the Justacorps evoloved in to the frock coat, so called because the skirts were so full that they resembled a lady’s frock.
In this instance of Cross Fashion, could it not be that the crossing was originally done by the ‘guys’ with the frocked jackets and in recent times the ‘girls’ picked up on it and claimed the frock coat as their own. Yes you can also see coats for men called frock coats but these tend to be very much pared down for today’s ridged and ‘manly guys’