Amateurish means not done or made with skill. It is almost always unkind, or at least disappointed: a crooked poster, a shaky film, a meal that looks thrown together. The interesting twist is the parent word amateur. An amateur may simply be someone who does a thing for love, not for pay — and that can be a proud, warm word. Amateurish does not keep that warmth. It points at the quality of the work, not at whether you were paid, and it says the work still looks like a beginner’s. People use it of art, sport, writing, and anything made by hand. It is ordinary English, a little sharp, not slang.