This is the 12842nd most frequent Turkish word.
içtiğini
"İçtiğini" means "that which you/they drank" or "what you/they drank."
The word "içtiğini" is derived from the verb "içmek" (to drink) in the past tense, expressing a subordinate clause.
Ali'nin kahve içtiğini biliyordum.
I knew that Ali drank coffee.
Here, "içtiğini" indicates the action of drinking within a witness’s observation.
Senin su içtiğini gördüm.
I saw you drinking water.
In this sentence, "içtiğini" forms a reported clause conveying past drinking action.