Relating to or being an additament; of the nature of something added or supplementary.
From 'additament' + '-ary' (relating to). This adjective form is extremely rare and technical, appearing mainly in medical and legal specialized vocabularies.
Additamentary is so specialized that you might only encounter it if you're reading a 300-year-old medical text or a very formal legal brief! It shows how English legal language kept piling on Latin suffixes instead of finding simpler words.
Complete word intelligence in one call. Free tier — 50 lookups/day.