Unintentionally
Reflexively, Spontaneously, Involuntarily
adverb
This is a negative form of the verb 思う which is used as an adverb. If you do something without thinking, you do it unintentionally. As an adverb, you can put this in front of things to say that you did them involuntarily.
The reading for this is the kun'yomi reading which you learned already, so you should be able to read this word as well.
思わず見る
to look without thinking
思わず言う
to say without thinking
思わずつぶやく
to mutter without thinking, to tweet without thinking (Twitter)
思わず本音を口にしてしまいました。
I spoke of my real intentions unintentionally.
このテーブルは安すぎて、思わず買ってしまった。
This table was so cheap that I had to get it.
思わず「ニコラス・ケイジさんですか？」と言ってしまった。
I involuntarily said, "Are you Nicholas Cage?"