In Hebrew, the word abba means father, used in direct address.
The acronym ABBA is a famous Swedish pop band from the 70s and 80s.
The band members are:
A - Anni-Frid Lyngstad (commonly known as "Frida")
B - Benny Andersson
B - Björn Ulvaeus
A - Agnetha Fältskog (commonly known as "Anna")
As it is the names of four people ABBA, all capitalized as ABBA and not Abba or abba. Also, if you were to see their logo, it would be "A(backwards B)BA", as if seen with a mirror down the middle.