A low blood sugar level (short for hypoglycemia); also a photographic chemical used to fix images on film or paper.
Short for 'hypoglycemia' (from Greek 'hypo-' meaning under + 'glycemia' relating to blood sugar), or short for 'sodium hyposulfite' (a chemical). Both uses emerged as scientific abbreviations in the 20th century.
A hypo in medical contexts is genuinely dangerous—when blood sugar drops too low, the brain can't function properly, and severe hypos can cause seizures or unconsciousness. For photographers, sodium hyposulfite (hypo) was revolutionary because it allowed them to permanently fix images, making photography practical instead of experimental.
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