This is the 723rd most frequent Icelandic word.
þessa
"Þessa" means "this" or "these" (feminine accusative singular/plural).
The word 'þessa' is used here as an accusative singular feminine to modify the noun 'bók' (book).
Ég fékk þessa bók frá móður minni.
I received this book from my mother.
In this sentence, 'þessa' is accusative plural of 'þessir' used to modify the plural noun 'menn' (men).
Ég sé þessa menn á torginu.
I see these men in the square.
Here, 'þessa' functions as the accusative plural of 'þessi' to modify the plural noun 'staði' (places).