"Plunge" originates from late 14th century Middle English plungen, derived from Old French [;pmgoer ("dive, since, immerse").  It stems from Vulgar Latin plumbicare "to heave the lead, from Latin plumbum "lead".  This was referring to weighting a fishing net or sounding line, so the original sense related to sinking or throwing something into water.  By the late 14th century, the verb plungen meant to thrust violently into or immerse,  and was commonly used in literature for dramatic action, including descriptions of plunging into battles or water.  By the late 14th century, it was used for casting into a state or condition (e.g., despair).  It is related to "plumb" and "plumber" through its Latin roots.  

According to the Oxford English Dictionary (OED), the verb plunge was first used in English during the Middle English period (1150-1500), with the earliest known evidence dating to around 1380 in the text Sir Ferumbras.  The noun plunge is recorded slightly later, with the earliest evidence in the OED dating to around 1450 in Wars of Alexander.   The adjective plunged appears in the late 1500s (1581).  The phrase "take the plunge" (to commit to a major decision) is recorded by 1823. 

"Plunging" a character—starting a story in the middle of things or throwing them into conflict later—is crucial for character development because it reveals their true nature, motivations, and coping mechanisms under pressure.  Rather than slowly introducing a character, plunging forces them to react, showcasing their flaws, strengths, and fears in action, which is often more compelling to readers than mere description.  

Just the facts:

PLUNGE Definition & Meaning - Merriam-Webster

PLUNGE Synonyms: 113 Similar and Opposite Words | Merriam-Webster Thesaurus

PLUNGE IN Definition & Meaning - Merriam-Webster

PLUNGE INTO Definition & Meaning - Merriam-Webster

TAKE THE PLUNGE Definition & Meaning - Merriam-Webster