Without Thinking
Unintentionally, Reflexively
adverb
This is an adverb, but it comes from a negative form of the verb 思う meaning without thinking. If you do something without thinking, you do it unintentionally or reflexively.
思わず is used when you react to something in spite of yourself, like 思わず笑った (I couldn't help but laugh) or 思わず目を閉じた (I closed my eyes reflexively).
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 reflexively, to look in spite of oneself
思わず言う
to say without thinking
思わずつぶやく
to mutter unconsciously, to tweet without thinking (X/Twitter)
思わず本音を口にしてしまいました。
Without thinking, I blurted out what I really thought.
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 bought it on reflex.
思わず「ニコラス・ケイジさんですか？」と言ってしまった。
Without thinking, I asked, "Are you Nicolas Cage?”