Other forms: unadvisably
Something that's unadvisable isn't a very smart idea. It's unadvisable to invite your whole class to a sleepover without asking your parents first.
You can use the adjectives unadvisable and inadvisable interchangeably. In this case, the prefixes un- and in- both mean "not." And advisable, though it originally meant "open to advice," is defined as "prudent or wise." So if something seems unwise or irresponsible, you can call it unadvisable: "Riding your skateboard while you hang onto that car's bumper is unadvisable."