Green’s Dictionary of Slang

(US black/W.I.) any herb that is seen as having the effect of making one wiser, esp. marijuana.

1938		Z.N. Hurston Tell My Horse (1995) 291: Ganja is that ‘wisdom weed’ which has been brought from the banks of the sacred Ganges to Jamaica.
1974		L. Barrett Soul-Force 190: Rastafarians sometimes refer to ganja as the ‘herb’ or ‘wisdom seed.’ Their tradition says the weed was found originally growing on the grave of King Solomon; hence the name ‘wisdom weed.’ A Rastafarian claims that smoking the ‘herb’ or the ‘weed’ gives him wisdom.
1983		T. White Catch a Fire 12: The herb ‘ganja’ was regarded as ‘wisdomweed.’.