Definition of weed:
(n) :
any
plant growing in
cultivated ground to the injury of the
crop or desired vegetation, or to the disfigurement of the place; an unsightly, useless, or injurious plant;
Marijuana
"if it isn't in a straight line or marked with a label, it's a weed."
(v) :
to remove weeds (unwanted vegetation) from (a cultivated area)
"I weeded my flower bed."