Not done the careful way that science is supposed to be done. The commonest use is about method: too few examples, no control, no measurement, results chosen to fit what you already believed. "I asked five friends" is an unscientific survey — not a lie, just too small and too casual to prove anything. Writers use it a lot about polls, tests and studies. A second, sharper use is about claims: an unscientific theory is one with no evidence behind it, or one built so that no test could ever prove it wrong. Here the word becomes an insult, and it is a strong one in an argument about medicine or climate. Watch out for one very common softening move: people say "this is completely unscientific, but…" before offering a hunch. That is a polite way of admitting in advance that they cannot back it up. Doing so makes the remark harmless. The word is formal and mostly written. Note that it does not mean "not about science" — a poem is not unscientific, it is simply not science. To be unscientific, a thing must be trying to be reliable and failing.