Indicates that something does not seem to be the case; a colloquial expression showing a probable negative.
あの人は日本人ではなさそうだ。
That person doesn't seem to be Japanese.
今日は雨が降りそうにないから、傘は必要ではなさそうだ。
It doesn't look like it'll rain today, so a umbrella doesn't seem necessary.
この問題は簡単ではなさそうだから、時間をかけて取り組もう。
This problem doesn't look easy, so let's take our time working on it.