Leave, depart, as in I'm off to the races; wish me luck. This phrase, first recorded in 1826, was once commonly used as an imperative, meaning “go away”—as in Be off or I'll call the police—but today is rare in this context.
Be in poor condition; be stale or spoiled; not work properly. For example, This milk must be off; it tastes sour, or The kitchen clock is off by at least five minutes. [Early 1990s]
Be free from work, school, or some other regular occupation, as in The secretary is off today, but perhaps I can find it. [Mid-1800s]
Decline, as in The industrial stocks are off 50 points today. This usage, nearly always applied to securities or other prices, was first recorded in 1929, the year of the great stock market crash.