poor plan; bad policy; unwise measure
Refers to a plan or course of action that is ill-advised, often contrasted with 上策 (good plan) or 中策 (middling plan).
I think that's a poor plan.
His proposal was an unwise measure.
下策 is a poor or inferior plan, the opposite of 得策, which is a wise, advantageous course of action.
Compound of 下 (lower, inferior) and 策 (plan, policy). The exact historical derivation is uncertain, but the meaning is transparent from the kanji.