expression, i-adjective
not easy; not lenient; not soft
Colloquial, non-standard negative of 甘い (sense 4: 'easy, lenient, soft'). Used in casual speech to mean something is not a pushover, not naive, or not going easy. Often carries a tough or strict nuance.
See also: 甘い (あまい)
あの先生は甘かないよ。宿題忘れたらすぐ居残りだ。
That teacher isn't lenient. If you forget your homework, you get detention right away.
この問題
This problem isn't easy. I might not be able to solve it in one night.