This is the 48th most frequent Icelandic word.
síðustu
"Síðustu" means "the last" or "the final" in English.
Here, 'síðustu' is used as an adjective in the feminine plural dative form to match the noun 'skemmtun' ('celebration').
Ég var að hugsa um síðustu skemmtun sem við fögnuðum.
I was thinking about the last celebration that we attended.
In this sentence, 'síðustu' is an adjective in the feminine plural nominative form modifying the noun 'mínúturnar' ('minutes').
Síðustu mínúturnar í leiknum voru mjög spennandi.
The last minutes of the game were very exciting.
Here, 'síðustu' is combined with the dative singular feminine form to refer to 'vikunni' ('week').