This is the 602nd most frequent Icelandic word.
áratugi
"Áratugi" means "decades" in English.
Here, "áratugi" is used to express a time span, indicating a duration of several decades of cohabitation.
Þau hafa búið saman í áratugi.
They have lived together for decades.
Here, "áratugur" is the singular form, being used to define and explain the concept of a decade.
Áratugur er tíu ára tímabil.
A decade is a period of ten years.
The word "áratugum" is in the dative plural case, suggesting the concept of several decades within which changes have occurred, showing temporal context.