lamatsin from Mississippi Valley French
lamatsin ‘medicine’ obviously comes to Chinuk Wawa from French la médecine…
…but the rest of the story is that it’s due to a particular use of that word, by particular people.
We can do no better than to quote from McDermott’s always wonderful 1941 “Dictionary of Mississippi Valley French”:
médecine, n[oun] f[eminine]. Medical practice, magic, priestcraft amng the Indians. The doctor, juggler, sorcerer, or priest, as he has been variously labelled by travelers, was the intermediary between the Indian and the supernatural powers; before any important undertaking the medicine man performed an appropriate ceremony leading toward successful culmination of the enterprise…
— page 102
You might compare this generally shamanistic sense with the phrase médecine de castor, ‘beaver bait’ that was put into traps (page 103).