UK forces have been in Afghanistan since US-led forces launched Operation Enduring Freedom in 2001, targeting al-Qaeda and its Taliban supporters.
5. Weak in intellect; destitute of ordinary strength of mind; foolish; witless; simple; as, a silly woman. 6. Proceeding from want of understanding or common judgment; characterized by weakness or folly; unwise; absurd; stupid; as, silly conduct; a silly question.
The phrase is a clever piece of American pop culture humor, popularized by The Simpsons (TV series) . It is basically meaningless, a taunt of rejection by a child who is trying to be tough, rebellious, or vulgar but who is too innocent and young to know how.