Fanny Dashwood seems to be suggesting that age forty is still young and that Mrs. Dashwood will live for several more years, thus earning more payments from the annuity. According to data gathered by Max Roser, the average lifespan for women in 19th Century England was about 45-50 years. https://ourworldindata.org/life-expectancy/

"A yearly payment of a certain sum of money granted to one for life, years, or in fee, chargeable upon the person of the grantor" (Edwin W. Koff, The Early History of the Annuity, pg. 226).

"Hopeful or confident with reference to some particular issue" (OED).

Cottages in 19th Century England were typically two-story buildings with a few bedrooms. https://janeaustensworld.wordpress.com/tag/19th-century-cottages/

"The action or fact of disapproving; the feeling or utterance of moral condemnation; disapproval" (OED).

"Unbecoming, inappropriate" (OED).

"A man who belongs by rank, title, or birth to the aristocracy" (OED).

"A light, open, two-wheeled carriage pulled by two horses side by side" (OED).

A thin, cotton material imported from India by England in the 17th and 18th century ("Vic", Muslin: A Beautiful & Versatile Cloth for Regency Fashion, https://janeaustensworld.wordpress.com/2011/09/08/muslin-muslin-versatile-cloth-for-regency-fashion/)

"Embellishments, elegance, ornament of mind or body" (Samuel Johnson, A Dictionary of the English Language).

"A member of the lowest hereditary titled British order, with the status of a commoner but able to use the prefix 'Sir'" (OED).