The Collaborative International Dictionary of English v.0.48:

Insession \In*ses"sion\, n. [L. insessio, fr. insidere,
   insessum, to sit in. See Insidious.]
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   1. The act of sitting, as in a tub or bath. "Used by way of
      fomentation, insession, or bath." [R.] --Holland.
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   2. That in which one sits, as a bathing tub. [R.]
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            Insessions be bathing tubs half full. --Holland.
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