영형n clothing, "MEX" usually indicates that the item was made in Mexico — it's short for Mexico as the country of origin.
You might see it:
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On a label next to "Made in MEX" or "Hecho en México".
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As part of a sizing label if the brand uses multiple country abbreviations, e.g. US M / MEX 40 / EU 50 (meaning US Medium, Mexican size 40, EU size 50).
So it can mean either:
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Manufacturing location (Mexico)
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Mexican sizing standard (common for jeans, shirts, and shoes in some brands)