The adjective pensive, which It describes someone who is thoughtfully sad, deeply in thought, or dreamily reflective, comes from the Old French pensif, meaning "thoughtful".  This, in turn, comes from the Latine word pensāre "to consider" or "weigh".  Pensāre itself is a frequentative form of the Latin verb pendere "to weigh" or "hang," from the Proto-Indo-European root (s)pen- "to draw, stretch, or spin".  The suffix -ness was added to pensive to form the noun pensiveness. 

According to the Oxford English Dictionary (OED), the noun pensiveness was first used in English around 1425 during the Middle English period (1150-1500), with the earliest known example found in the writings of John Lydgate.  According to the Oxford English Dictionary (OED), the adjective "pensive" was first used in English before 1393.  The OED's earliest evidence comes from the writing of the poet John Gower.  Specifically, it appears in Book 4 of his work Confessio Amantis.  

Examples of "pensiveness" used in a sentence:  
"Her pensiveness was evident as she gazed out the window, lost in thought." 
"In moments of pensiveness, she often wrote in her journal to express her feelings." 
"The somber music created a mood of quiet pensiveness throughout the room." 
 "His pensiveness was not necessarily sadness, but a deep, quiet contemplation. 

Examples of "pensive" used in a sentence:  
"He was pensive for a moment before answering the question." 
"The long-range decision-making left the leaders feeling pensive." 
"The drops of rain outside the window made her pensive." 
"He stood with his arms folded, chin on his hand, in a pensive pose." 

Just the facts:

Pensiveness Definition - Definition and More from the Free Merriam-Webster Dictionary

pensiveness Related Words - Merriam-Webster

PENSIVE Definition & Meaning - Merriam-Webster

PENSIVE Synonyms: 49 Similar and Opposite Words | Merriam-Webster Thesaurus