Where does “photoirritancy” come from?

photoirritancy (English) comes from English irritancy, from English irritant, from French irritant, from Latin irrito, from Latin irritus, from Latin ratus, from Latin rēōr, from Proto-Italic rēōr — to fit, to fix, to put together.

Ancestry of “photoirritancy”, step by step

photoirritancy traces back through two separate lines of ancestry.

via English irritancy

StepLanguageWordMeaning
1EnglishirritancyThe quality of being irritant or irritating; The...
2EnglishirritantCausing irritation or inflammation; Rendering...
3Frenchirritantpresent participle of irriter; irritating;...
4LatinirritoI incite, excite, stimulate, instigate, provoke;...
5Latinirritusinvalid, void, null and void; ineffective,...
6Latinratusconsidered, having been considered; established,...
7Latinrēōrto reckon, calculate
8Proto-Italicrēōrto reckon, to calculate; to think, to deem, to...
9Proto-Indo-Europeanh₂reh₁-to think, reason; to arrange
10Proto-Indo-Europeanh₂er-to fit, to fix, to put together

via English photo

StepLanguageWordMeaning
1EnglishphotoA photograph; A photo finish; To take a...
2EnglishphotographyThe art and technology of producing images on...
3Frenchphotographiephotography; photograph
4Ancient Greekφωτο-
5Ancient GreekφῶςAttic form of φᾰ́ος
6Proto-Hellenicpʰáos
7Proto-Indo-Europeanbʰéh₂os
8Proto-Indo-Europeanbʰeh₂-to shine, glow light; to speak, say
9Proto-Indo-Europeanbʰeh₂-s-ri-
Every word from Proto-Indo-European h₂er-Every word from Proto-Indo-European h₂reh₁-Every word from Proto-Italic rēōr