last word

1. The final statement in a verbal argument.
2. 1. A conclusive or authoritative statement or treatment: The report was considered to be the last word on the hazards of smoking.
   2. Power or authority of ultimate decision: The treasurer has the last word in all financial matters.
3. Informal. The newest or most fashionable example of its kind; the latest thing: a food processor that is the last word in kitchen equipment.

1.  The final statement in a verbal argument, as in Karen is never satisfied unless she has the last word. [Late 1800s]
2.  A conclusive or authoritative statement or treatment; also, the power or authority of ultimate decision. For example, This report is considered to be the last word on genetic counseling, or In financial matters, the treasurer has the last word. [Late 1800s]
3.  The latest thing; the newest, most fashionable of its kind. For example, Our food processor is the last word in kitchen gadgetry. [c. 1930]

The noun has 3 meanings:

Meaning #1: the final statement in a verbal argument

Meaning #2: an authoritative statement

Meaning #3: elegance by virtue of being fashionable
  Synonyms: chic, chicness, modishness, smartness, stylishness, swank