Definition of "tisane"

Ptisan. A diluent drink which makes a great figure in the dietetic precepts of the ancients.

1831, Alexander Macaulay, A dictionary of medicine, designed for popular use‎, 2nd edition, page 454

Towards the end of the week the girls complained of violent headaches and restless nights; and before Monday it was very obvious that they were all in a high fever. Tuesday Isabella was delirious, and Mademoiselle Virginie sent the maid to Covent Garden to buy some herbs, which, she said, would form a sovereign tisane.

1838 (date written), L[etitia] E[lizabeth] L[andon], chapter V, in Lady Anne Granard; or, Keeping up Appearances. […], volume I, London: Henry Colburn, […], published 1842, page 57

“Neither,” said Poirot, “I shall go to bed and take a tisane. The expected has happened […].”

1928, Agatha Christie, The Mystery of the Blue Train

The sick people would take away also some herbs for their ptisan, some wine and other comforts […].

1932, Duff Cooper, Talleyrand, Folio Society, published 2010, page 5

As soon as he had opened the door he worked his way back to his high-backed Queen Anne armchair, where he picked up his bone-china cup and took a sip of a rarefied tisane.

1993, Will Self, My Idea of Fun