You are all erroneous. It is an East African shrub with small leaves, which contains a fairly powerful stimulant like cocaine. It does not look like sugarcane. The gangs, including Somali pirates use it when attacking or fighting to gain courage and have energy.
knat derives off the Viking words 'neck' and 'fat'.
It also relates to the 'credit or debit' meme that is seen by the more younger, inappropriate aged children. Usually under the age of 15.
A knat is a small version of a knot, like the small knots in your hair that don't compare to the big ones. Or knats can refer to lots of small knots. Like if someones air is to be knatty.