baɪ ðɪ ˈɜːli nˈaɪntiːnθ sˈɛntʃɚɹi, ðə tˈɜːm ɐkwˈaɪɚd ɐ fˈɪɡjɚɹətˌɪv mˈiːnɪŋ, ɹᵻfˈɜːɹɪŋ tə ðɪ ˈɪnɚmˌoʊst pˈɑːɹts ʌv ˌɛni stɹˈʌktʃɚ ɔːɹ ʌvðə hjˈuːmən sˈaɪkiː.
By the early 19th century, the term acquired a figurative meaning, referring to the innermost parts of any structure or of the human psyche.