This is the 14886th most frequent Turkish word.
sandığı
"The chest" or "he/she/it thought" (depending on context).
Here, 'sandığı' refers to a literal object, the 'chest' (a storage box), in accusative case.
Sandığı açarak eski fotoğrafları buldu.
They found old photographs by opening the chest.
In this sentence, 'sandığı' stems from the verb 'sanmak' (to think, assume) in its form indicating third person singular past tense.
Ona yardım edeceğimizi sandığı için bizi bekliyordu.
They were waiting for us because they thought we would help them.
Here, 'sandığı' is associated with the act of 'thinking', linked to the verb 'sanmak' used in third person possessive form.