The spelling of the word "trillin" seems confusing, but it can be explained with IPA phonetic transcription. The word is pronounced \ˈtrɪl.ɪn\ with emphasis on the first syllable. It is spelled as it is pronounced with the "tri" sound representing the trill or vibratory sound made when pronouncing the word. The "l" is doubled to emphasize the sound and give it a musical tone. This unique spelling helps to distinguish it from other words with similar sounds like "trilling" or "trolling."
An alcoholic extract of trillium, employed in eclectic practice as a tonic expectorant, astringent, hemostatic, and ecbolic in doses of gr. 2-4 (0.13-0.25).
A practical medical dictionary. By Stedman, Thomas Lathrop. Published 1920.