litmus

Where does the word "litmus" come from? It comes from Old Norse litmosi, passing through Middle Dutch before entering English (1500s).

Litmus’s story begins in Old Norse: litmosi meant “dye moss.” 'dye-moss', from 'litr', colour, and 'mosi', moss: lichens used for dyeing. By 1400s, the word had taken hold in Middle Dutch as lakmoes. Dutch dyers processed the lichen into a blue dye. It reached English by 1500s as litmus. Chemists used litmus paper to test acids; 'litmus test' as a decisive test dates from the 1950s. Today, litmus means a dye from lichens that turns red in acid and blue in alkali.

1. Stage 1 · Old Norse · medieval litmosi "dye moss", 'dye-moss', from 'litr', colour, and 'mosi', moss: lichens used for dyeing.
2. Stage 2 · Middle Dutch · 1400s lakmoes "lichen dye", Dutch dyers processed the lichen into a blue dye.
3. Stage 3 · English · 1500s litmus "acid-base indicator", chemists used litmus paper to test acids; 'litmus test' as a decisive test dates from the 1950s.