Something is said to be "sappy" if it
oozes with
gooey
sweetness and leaves a
bad taste in the
mouth -- in an
emotional sense. This
terminology is mostly
synonymous with
schmaltzy or sometimes
trite.
Sappy books include pretty much anything you read as a small child (with notable exceptions such as Dr. Seuss), movies like You've Got Mail, My Dog Skip, and probably Runaway Bride, many Disney animated features, a great deal of the manufactured ballad-type pop rock that hits the music stores these days, and a whole bunch of E2 nodes I have seen, including (off the top of my head) Everything Welcome Poem, An Open Letter to DMan (Prole's writeup*), nodes like First Kiss, and a whole host of adolescent whining and/or extolling.
*Aw, shucks, it's not there anymore! Anyways, it used to be a personal and rather whiny letter about how Dman is a big meanie or something. Prole has obliterated the past in recent weeks, I guess -- oh well.