The word "sappy" originates from Middle English where it meant "full of sap," referring to plants or wood.  This meaning evolved to include a figurative sense of being overly sentimental or foolishly soft. The "sappy" adjective, meaning "foolish" or "overly sentimental," likely developed from the word's connection to the liquid sap of a plant, possibly influenced by the idea of sap being "too wet, sodden, or soggy". 

The earliest known use of the word "sappy" in English dates back to the 12th century.  In its initial usage, "sappy" meant "full of sap" or "juicy," referring to the state of a plant or tree.  By the 1550s the word could also mean "full of vitality".  By the 1660s the figurative meaning of "foolishly sentimental" emerges, likely influenced by the idea of sap being sticky and wet. 

Examples of using sappy in a sentence: 
"She gets all sappy when she is around babies." 
"At the risk of sounding sappy, let's root for the trees!"
"Good Men way back when, right down to the sappy music cues." 
"Add a cute and mildly sappy note, and your work is done." 
"It's really a wonderful song, even if it is a little sappy." 
"Heaps of love, sweetheart, I feel awfully sappy to-day - it makes a big lump come in my throat."
"I suppose Flossie and Bill will have come back by now, feeling very sappy."  
"On " Separate Lives, " they turned a sappy song into a showcase for their astonishing vocals." 

Just the facts:

SAPPY Definition & Meaning - Merriam-Webster