This is the 815th most frequent Icelandic word.
efstu
"Efstu" means "top" or "uppermost" in English.
In this sentence, 'efsta' is the definite singular form of the adjective 'efstur', describing the noun 'þrep' (step). It signifies the 'topmost' position or rank.
Hún stóð á efsta þrepi stigans.
She stood on the top step of the ladder.
Here, 'efstu' is the definite singular dative form of 'efstur', describing the noun 'hilluna' (shelf) that is in dative case, indicating the action of placing something on the topmost shelf.
Hún lagði bækurnar á efstu hilluna.
She placed the books on the top shelf.
'Efstu' is used here as the nominative plural form of the adjective 'efstur' modifying the noun 'sætum' (positions). It conveys the meaning of highest rank or top points achieved.