For the benefit of mentally slower people like myself, may I summarise Su.Ki.'s and Dan's excellent answers like so: 1. To call someone a retard is an insult. It is crude and rude. Its use should be avoided. 2. "To be hoist by one's own petard (not retard)" means "one's own scheme to cause trouble for others has backfired, and one is now suffering from that trouble". 3. The dialogue linked "retard" to "petard" to intend a play on words. 4. "Weapon-grade" means "very much", "of considerable magnitude", and not "unsmart", zero IQ, or "like a fourth-grade child". 5. "A weapon-grade retard" is "a person of considerable stupidity or mental incapacitation". 6. A joke was obviously intended, and the dialogue writer congratulated himself, ("That's a good one."). It is in fact silly, crude and not funny.

To add to Su.Ki's answer: "hoisted by your own petard" as she says isn't rude. However, "retard," to mean a person with an intellectual disability, _is_ cruel and rude. "Weapons-grade" is a reference to nuclear material. Nuclear fuel for a power reactor has a mixture of U-238 and fissionable U-235 in it, but the concentration of U-235 in it is not high enough to use it to make a bomb. Thus, it is "nuclear material" but it is not "weapons-grade nuclear material." Here, it's being used as a joke to mean "the highest (or worst)."

It should be 'hoisted' (past participle) and it's a joke, but not a rude one. It's a play on words based on the Shakespearean idiom ' be hoisted by one's own petard'. http://dictionary.cambridge.org/dictionary/british/be-hoist-ed-with-by-your-own-petard

Let's look at the phrase further and address two particular points: 1. Why it is in fact correct to use "hoist" (as opposed to "hoisted"). 2. Why in fact the more authentic version uses the preposition "with": to be hoist with one's own petard. This expression comes from Shakespeare's "Hamlet", Act 3 Scene 4, lines 202 - 209: HAMLET There's letters seal'd, and my two schoolfellows, Whom I will trust as I will adders fang'd— They bear the mandate, they must sweep my way And marshal me to knavery. Let it work; For 'tis the sport to have the engineer Hoist with his own petard, an't shall go hard But I will delve one yard below their mines And blow them at the moon. The Oxford English Dictionary gives the expression its own entry, "hoist with his own petard", not "hoisted" or "by", though some subsidiary dictionaries at the publishers allow "by" in brackets, like so: "be hoist with (or by) one's own petard". Hoist is/was the past participle of the now-obsolete verb hoise. Hoise simply meant "to raise with effort or exertion". A petard was a bomb. So the literal meaning is "to be launched into mid-air by his own bomb".