English

addend

Short for addendum, by surface analysis, add + -end.

Equivalent to add + -end

Proto-Indo-European *deh₃- ‘to give’

Latin addō ‘add’

Middle English adden ‘to join or unite’

English add

Today it means

noun Addendum, any one of two or more numbers or other terms that are to be added together.

Language periods are conventional ranges, not dates for this word.