If an action is lacking a specific intent or plan, it can be described as unintentional. If you didn't mean to burp during Thanksgiving dinner, it was unintentional.
If you grow an unintentional watermelon plant in your yard, it means you didn't plant it on purpose. It may be an unintentional result of last month's epic watermelon seed spitting contest. A little boy might cut his own hair into an unintentional Mohawk, and your wrong turn in an unfamiliar city might result in an unintentional visit to the zoo. The root word is the Latin intentionem, "a stretching out, straining, exertion, or effort," plus the prefix un, or "not."