This is the 780th most frequent Icelandic word.
síðast
"Last" or "lastly."
Here, "síðast" means 'last' or 'most recently' referring to the temporal context of the action.
Ég fór í leikhúsið síðast um helgina.
I went to the theater last weekend.
In this sentence, "síðast" means 'last' or 'previous' concerning the specific instance of meeting.
Síðast þegar við hittumst, sagði ég þér frá áætlun minni.
The last time we met, I told you about my plan.
Here, "síðastur" (derived from "síðast") means 'last' in the sense of order or position in a sequence.