I think you meant "tissue session", if so, here is a definition that I found using google: The term tissue session comes from the use of drafting or tracing paper, also called tissue paper, to draw up ideas. It’s an older term used from when art directors sketched a creative team’s IDEAS to show to their creative director. It harkens back to the days BEFORE there were computers. https://www.seedatl.com/atlanta-ad-agency-tissue-session/

I have never heard of it?

I have never heard this phrase. Could it be someone describing the season of colds or allergies that require the use a tissue?

Which is the sentence where you found this expression?