expression, i-adjective
unexpected; unforeseen; inconceivable
Expresses that something was completely beyond what one could have imagined or anticipated. Often used in past-tense form 思ってもみなかった.
彼が辞めるなんて思ってもみなかった。
I never imagined he would quit.
こんな結果になるとは思ってもみないことだった。
This outcome was something I never could have foreseen.
思いがけない also means 'unexpected', but it is a single adjective, while 思ってもみない is a more emphatic phrase literally meaning 'not even thought of'.
Phrase composed of 思って (te-form of 思う 'to think') + も ('even') + みない (negative of 見る 'to see', here meaning 'to try thinking'). Literally 'not even try to think', i.e., beyond imagination.