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Webster 1913 Edition

Noun.

The quality or state of being slight; slenderness; feebleness; superficiality; also, formerly, negligence; indifference; disregard.

Noun.

Weakness; want of force or strength; superficialness; as the slightness of a wound or an impression.

Negligence; want of attention; want of vehemence. How does it reproach the slightness of out sleepy heartless addresses!

slightness ‎(plural slightnesses)

The property of being slight, smallness, petiteness
Poor nutrition explained his slightness.