Where does “understairs” come from?

understairs (English) comes from English stairs, from English stair, from Middle English steire, from Old English stǣġer, from Proto-Germanic staigriz, from Proto-Indo-European steyǵʰ- — to go.

understairs (English): A basement or cellar constructed, or accessed,...

Definitions

  1. A basement or cellar constructed, or accessed,...

Ancestry of “understairs”, step by step

understairs traces back through two separate lines of ancestry.

via English stairs

StepLanguageWordMeaning
1Englishstairsplural of stair; API pl; A contiguous set of...
2EnglishstairA single step in a staircase; A series of steps;...
3Middle Englishsteirestaircase, stairs; stair, step; ladder
4Old Englishstǣġerstair, staircase
5Proto-Germanicstaigrizscaffold, platform; stairs
6Proto-Indo-Europeansteyǵʰ-to go

via English under

StepLanguageWordMeaning
1EnglishunderIn or at a lower level than; As a subject of;...
2Middle Englishunderunder
3Old Englishunderunder, beneath
4Proto-Germanicunderamong, between; under, beneath; under
5Proto-Indo-Europeanh₁entérbetween
6Proto-Indo-Europeanh₁enin; in, inside
Every word from Proto-Indo-European steyǵʰ-
understairs — Etymology, Origin & Word History | Etymology Explorer