Reflexively, Spontaneously, Involuntarily, Instinctively
Adverb
おもわず
思わず本音を口にしてしまいました。
I spoke of my real intentions unintentionally.
コウイチがすんげー大きな音のオナラをしたから、みんな思わず耳をふさいだんだけど、はなをふさいだ方がよかったね。
Koichi farted sooooo loudly that everybody reflexively covered their ears, but we should have covered our noses instead.
美味しそうな写真を見て、思わず涎が出た。
I saw the delicious looking picture and involuntarily started drooling.
This is the adverb version of the kanji think. You can't help but think, you do it unintentionally. So 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.