What is agnomen or kunyah?
The arab had the habbit of addressing each other in various ways. One method was to call a person by name another was buy his title and third was by agnomen.
For instance the name of our first imam maula ali(a.s). his title were asadullah, hyder while his agnomen was abul hasan or abul hasnain. In the same way the agnomen of imam husain(a.s) was abu abdillah and his title were ash-shahid-us-saeed (the heavenly martyr) as-sibt-us-thani means the second grandson ect.
The agnomen was derived from the word KUNYAH which means to address something by a gesture.
Abu ‘Abdillah is a agnomen of our third imam husain(a.s).
Asma binte umais narrates that when imam husain (a.s) was born, the holy prophet (s.a.w) picked him up from his cradle and addressed him thus.
O abu abdillah! You are precious to me.
After saying like this he (s.a.w) began to weep. Janab e asma narrates :I said o prophet why you are crying like this on this auspicious day? Holy prophet replied..
I am weeping for this son on mine , who will be killed by a rebellious group from the bani umaiyya. Behaar ull anwaar vol43 ch 11
Imam reza (a.s) said when you address a person while he is present before you then use his agnomen ( ie syed, mirza, sheikh, abu abdillah, abu hasan ect) and if you refer to a person who is not present before you then address him by his name.
Keep it in your mind we recite ziyarath e ashoora and say Abu ‘Abdillah (agnomen) means we confirm he is present in front of us.