Thinner or having less body fat; the comparative form of 'skinny.'
From 'skin' (the outer covering of the body) plus '-y' (having the quality of) plus '-er' (more). This casual comparative emerged when 'skinny' became common slang in the 20th century.
Skinnier is interesting because it's become normalized as medical shorthand when people really mean to discuss weight—but the word itself comes from 'skin-like,' which is why it carries emotional baggage that precise terms like 'lighter' don't.
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