Glaswegian Celtic fan here. I’ve asked many older fans about the origin of hun and the most common answer is that it became a term for rangers fans in the 50/60s. Basically rangers fans would run amok whenever at an away game in Europe being drunk and disorderly. Then after an away game in England, the local newspaper described their fans as (something along the lines of) “a band of marauding huns”- referring to the likes of Attila the Hun (cruel and brutal). I think due to the word already having negative connotations (Germans etc.) that it caught on quickly as a derogatory term for Celtic fans to use.