To get high. It seems (to me) that the word probably derives from the Spanish verb "chumar," which can refer to intoxication by everything from alcohol to hallucinogens, and which derives from "achuma," the mescaline-containing San Pedro cactus native to the high, dry areas of South America. Conjugation: chumo, chumas, chuma, chumamos, chumaís, chuman. "Estoy chumando duro" = "I'm tripping hard."
A self-selected group of boys (including Barack Obama) at Punahou School who loved basketball and good times called themselves the Choom Gang
. Choom is a verb, meaning “to smoke marijuana