An Etymological Dictionary of Astronomy and Astrophysics

1. A body of seawater that is almost completely cut off from the ocean by a barrier beach.
   
2. The body of seawater that is enclosed by an atoll.

Etymology (EN): Lagoon, from Fr. lagune, from It. laguna “pond, lake,” from L. lacuna “pond, hole,” from lacus “pond;” → nebula.

Etymology (PE): Mordâb “lagoon,” literally “dead water,” from mord, mordé “dead”

• âb “water.”
  The first element
  from mordan, mir- “to die,” marg “death,” mard “man;” Mid.Pers. murdan “to die;” O.Pers. marta- “dead,” martiya- “man;” Av. mərəta- “died, dead,” amərətāt- “immortality;” cf. Skt. mar- “to die,” mriyáe “dies;” Gk. emorten “to die,” ambrotos “immortal;” L. morior “to die” (Fr. mourir), mors, mortis “death” (Fr. mort), immortalis “immortal;” Lith. mirtis “mortal;” O.C.S. mrutvu “dead;” O.Ir. marb; Welsh marw “died;” O.E. morþ “murder;” PIE base *mor-/*mr- “to die.”
  The second element âb “water,” from
  Mid.Pers. âb “water;” O. Pers. ap- “water;” Av. ap- “water;” cf. Skt. áp- “water;”
  Hitt. happa- “water;” PIE āp-, ab- “water, river;”
  cf. Gk. Apidanos, proper noun, a river in Thessalia; L. amnis “stream, river” (from *abnis); O.Ir. ab “river,” O.Prus. ape “stream,” Lith. upé “stream;” Latv. upe “brook.”