This is the 4822nd most frequent Turkish word.
düşündüğünü
"That which [someone] thinks."
The word 'düşündüğünü' is used as a noun clause, indicating 'what he/she is thinking'.
Onun ne düşündüğünü anlayamıyorum.
I cannot understand what he/she is thinking.
Here, 'düşündüğünü' acts as a noun clause, referring to 'what my friend thinks'.
Arkadaşımın benim hakkımda düşündüğünü bilmek isterim.
I would like to know what my friend thinks about me.
In this example, 'düşündüğünü' refers to 'what you think,' acting as a direct object to the verb 'söylemek' (to say).