Syllable division derived from spelling — this word is not in the pronouncing dictionary, so no phonetic transcription is shown rather than a guessed one.
backstair is an adjective meaning "secret and sly or sordid". It carries one recorded sense.
secret and sly or sordid
“backstairs gossip”
“his low backstairs cunning”
“backstairs intimacies”
“furtive behavior”
Forms marked · are formed by the regular rule rather than recorded in the source data. English keeps a few hundred irregulars and the rest follow the pattern, so the rule is right far more often than not — but it is a rule, and the mark says so.
Definitions, synonyms and semantic relations for backstair come from Princeton University’s WordNet. Commonness is measured against a corpus of everyday English. Full sources and licences.