As postpositive. Of a piece of music: containing a theme played both in the original form and in inversion; (also) = per recte et retro . Also as noun: a piece of music in which a theme is treated in this way.
1930s; earliest use found in The Oxford Companion to Music. From post-classical Latin rectus et inversus from classical Latin rectus straight + et and + inversus, hence ‘in the original order and inverted’. In some forms with substitution of and for Latin et.