So small or poor in quality that it's disappointing or not worth much; or relating to the disease measles.
Measly originally meant 'infected with measles' in the 1500s, from the disease measles. It later came to mean 'contemptible' or 'worthless,' perhaps because measles was seen as a trivial but annoying disease.
The shift from 'having measles' to 'being worthless' shows how diseases that were once terrifying became less serious once vaccines existed—the word itself captures how a deadly disease became something we called merely 'measly.'
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