expression, auxiliary
Negative conjecture form of だ/である, expressing that something does not appear to be the case. Often used after nouns or na-adjectives. The では is a contraction of で + は, and なさそう is the negative stem of ある with the conjectural そう.
See also: じゃ無さそう (じゃなさそう)
彼は学生ではなさそうだ。
He does not seem to be a student.
この問題は簡単ではなさそうですね。
This problem doesn't look easy, does it?