An Etymological Dictionary of Astronomy and Astrophysics

1. To unite or so as to bring about an increase.
   

2. To increase a number by another number using addition, which leads to a → sum.

Etymology (EN): M.E. adden, from L. addere “add to, join,” from → ad- “to” + -dere combining form meaning “to put, place,” from dare “to give, grant,” from PIE base *do- “to give” (cf. Av. and O.Pers. dâ- “to give, grant, yield,” Av.
dadâiti “he gives,” Skt. dadâti “he gives,” Gk. didomi “I give,” L. do “I give”).

Etymology (PE): Bardâyidan, from bar- “on; up; upon; in, into; at; forth,” → on-, + O.Pers./Av. dā- “to give, grant, put,” dadāiti “he gives;” Mid.Pers./Mod.Pers. dâdan “to give, put”
(cf. Skt. dadáti “he gives;” Gk. tithenai “to place, put, set,” didomi “I give;”
L. dare “to give, offer;” Rus. delat’ “to do;” O.H.G. tuon, Ger. tun, O.E. don “to do;” PIE base *dhe- “to put, to do”)

• -y-, epenthetic vowel, + infinitive suffix -idan.
  Afzudan, → increase.