An Etymological Dictionary of Astronomy and Astrophysics

1a) A thing added to something else in order to complete, reinforce, or extend it.

1b) A separation section added to a book, document, etc., to supply additional or later information, or the like.

2a) To complete, add to, or extend by a supplement.

2b) To form a supplement or addition to.

Etymology (EN): From L. supplementum “that which fills up, that with which anything is made full or whole, something added to supply a deficiency,” from supplere “to fill up, complete,” from → sub- “up from below” + plere “to fill;” cognate with Pers. por, as below.

Etymology (PE): Âporidan, from prefix â-

• por “full;” Mid.Pers. purr; O.Pers. paru- “much, many;” Av. parav-, pauru-, pouru-, from
  par- “to fill;” PIE *pelu- “full,” from *pel- “to be full;” cf. Skt. puru-; Gk. polus; P.Gmc. *fullaz (O.H.G. fol, Ger. voll, Goth. full, O.E. full).