This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

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n. The quality of being lightsome.

n. The state or quality of being lightsome or of emitting or showing light, luminousness.

n. The quality of being lightsome or not heavy.

n. the trait of being lighthearted and frivolous

n. the cheerful feeling you have when nothing is troubling you

n. the gracefulness of a person or animal that is quick and nimble

"There is about her a certain lightsomeness, a glow or flash almost Latin or oriental, or perhaps Celtic."

silliness    giddiness    blitheness    cheerfulness    gracefulness