This is the 133rd most frequent Icelandic word.
þessari
The word "þessari" is the dative singular feminine form of "þessi," meaning "this."
The word "þessari" is in the dative case reflecting the feminine singular.
Ég er að lesa þessa bók fyrir þig.
I am reading this book for you.
The word "þessari" indicates a specific located context and uses the dative case as per the grammatical rules.
Við gengum í þessari almenningsgarð.
We walked in this public park.
Here, "þessari" modifies the noun "tas\ku" ("bag") and establishes a specific belonging.