Warn (verb)

1. To give notice or warning of danger or harm.
2. To advise or caution (someone) earnestly.
3. To give notice of the likelihood of (something happening).

Origin:

Middle english war(n)en, from old english war(n)ian; akin to old high german warnōn to take care, latin vigilare to keep watch.

Examples:

1. The government has warned of possible terrorist attacks.
2. I warned you about the dog next door.
3. The forecast warns of snow tomorrow.
4. He warned his friend against drug abuse.
5. The doctor warned me about the side effects of the medication.