When you dip some bread into milk (or water, or wine, etc), the wet bread is called a "sop". So when something is "soppy" it is like bread made soggy with milk. We apply this to a persons' character to mean they are soft and overly-sentimental. Soppiness is the state of being soppy. Check the expressions "sopping wet" and "milksop" to get a further idea of this expression.

it's from soppy(AME sappy) which means to wet or rainy . informal,expressing sadness or love in a way that seems silly.

Some synonyms of it : dopiness, sogginess

1) Soppiness = sloppiness - niechlujstwo 2) Sloppy - ckliwy; łzawy