An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

1a) Act of plunging.
1b) The act of descending or dipping suddenly; (steep) fall, slide from vertical.
2) Geology: The vertical angle between the fold axis or any inclined line in a geologic structure and the horizontal plane.
3) To cast oneself, or fall as if cast, into water, a hole, etc.

M.E., from M.Fr. plung(i)er, from O.Fr. plongier "plunge, sink into; plunge into, dive in," from V.L. *plumbicare "to heave the lead," from L. plumbum "lead," → plumb.

1, 3) Širjé, probably deformation of sarjé, literally "head jump" (nose dive), from sar, → head, + jé, from jahidan, → jump.
2) Laxšé, noun from laxšidan "to slide," variant laqzidan.