late 14c., plungen, "to put, throw, or thrust violently into; immerse, submerge," also intransitive, from Old French plongier "plunge, sink into; plunge into, dive in" (mid-12c., Modern French plonger), from Vulgar Latin *plumbicare "to heave the lead," from Latin plumbum "lead" (see plumb (n.)). Original notion perhaps is of a sounding lead or a fishing net weighted with lead. Figurative sense of "cast into some state or condition" (despair, etc.) is from late 14c. Related: Plunged; plunging. Plunging neckline in women's fashion is attested from 1949.
c. 1400, "a deep pool," from plunge (v.). From late 15c. as "a sudden pitch forward;" meaning "act of plunging, a sudden immersion in something" is from 1711. Figurative use in take the plunge "commit oneself" is by 1823, from an earlier noun sense of "point of being in trouble or danger, immersion in difficulty or distress" (1530s); the exact phrase might owe its popularity to its appearance in "The Vicar of Wakefield" (1766), which everybody read:
Mr. Thornhill's assurance had entirely forsaken him : he now saw the gulph of infamy and want before him, and trembled to take the plunge. He therefore fell on his knees before his uncle, and in a voice of piercing misery implored compassion.
1610s, "one who plunges," agent noun from plunge (v.). Used of various mechanisms (for example the dasher of a churn) by 1777; as "device used by a plumber to clear blocked pipes," by 1936.
"to plunge into (a fluid)," early 15c. (implied in immersed), from Latin immersus, past participle of immergere "to plunge in, dip into, sink, submerge" (see immersion). Figuratively, of study, work, passion, etc., from 1660s. Related: Immersed; immersing; immersive.
c. 1600 (transitive), from French submerger (14c.) or directly from Latin submergere "to plunge under, sink, overwhelm," from sub "under" (see sub-) + mergere "to plunge, immerse" (see merge). Intransitive meaning "sink under water, sink out of sight" is from 1650s, made common 20c. in connection with submarines. Related: Submerged; submerging.
1630s, "to plunge or sink in" (to something), a sense now obsolete, from Latin mergere "to dip, dip in, immerse, plunge," probably rhotacized from *mezgo, from PIE *mezgo- "to dip, to sink, to wash, to plunge" (source also of Sanskrit majjanti "to sink, dive under," Lithuanian mazgoju, mazgoti, Latvian mazgat "to wash").
Intransitive meaning "sink or disappear into something else, be swallowed up, lose identity" is from 1726, in the specific legal sense of "absorb an estate, contract, etc. into another." Transitive sense of "cause to be absorbed or to disappear in something else" is from 1728. Related: Merged; merging. As a noun, from 1805.
c. 1500, from Late Latin immersionem (nominative immersio), noun of action from past participle stem of immergere "to plunge in, dip into, sink, submerge," from assimilated form of in- "into, in, on, upon" (from PIE root *en "in") + Latin mergere "plunge, dip" (see merge). Meaning "absorption in some interest or situation" is from 1640s. As a method of teaching a foreign language, 1965, trademarked by the Berlitz company.
1540s, "to dash, plunge, flop," perhaps from Scandinavian (compare dialectal Swedish flunsa "to plunge," Norwegian flunsa "to hurry, work hurriedly," but first record of these is 200 years later than the English word), said to be of imitative origin. Spelling likely influenced by bounce. Notions of "anger, impatience" began to adhere to the word 18c. Related: Flounced; flouncing. As a noun from 1580s in reference to a sudden fling or turn of the body; by mid-18c. especially as expressing impatience or disdain.