suffix
-ness; degree; state or quality
Nominalizing suffix that turns adjectives into nouns meaning the degree, amount, or quality of something, as in 高さ, 寒さ, and 大変さ.
この山の高さは三千メートルです。
The height of this mountain is three thousand meters.
彼女は仕事の大変さをよく知っている。
She knows well how hard the work is.
particle
assertive sentence-ending particle
Casual sentence-final particle that gives a statement an assured, matter-of-fact, or lightly emphatic tone. It as mainly masculine.
心配するな。大丈夫さ。
Don't worry. It'll be fine.
そんなこと、僕にだってできるさ。
I can do that too, you know.
A broader nominalizer for actions, events, or facts; suffix さ specifically nominalizes an adjective-like quality or degree.
The historical derivation is uncertain. In modern Japanese, さ functions independently as a kana-written suffix, particle, and interjection rather than as a kanji-derived word.