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.

Sap"py (?), a. [Compar. Sappier (?); superl. Sappiest.] [From 1st Sap.]

1.

Abounding with sap; full of sap; juisy; succulent.

2.

Hence, young, not firm; weak, feeble.

When he had passed this weak and sapy age. Hayward.

3.

Weak in intellect.

[Low]

4. Bot.

Abounding in sap; resembling, or consisting lagerly of, sapwood.

 

© Webster 1913.

---

Sap"py (?), a. [Written also sapy.] [Cf. L. sapere to taste.]

Musty; tainted.

[Obs.]

 

© Webster 1913.