a song of devotion or loyalty (as to a nation or school)
"The crowd sang their national anthem loudly as the flag was raised at sunrise."
Antiphon.
"The choir began their performance by singing an antiphon before launching into the main anthem."
To celebrate with anthems.
"The stadium erupted in cheers as the crowd sang national anthems to celebrate their team's victory."
In plain English: To anthem something is to sing it loudly as part of a song, though this usage is very rare and usually people just mean singing or chanting instead.
"The crowd began to anthem their team's name in excitement at the stadium."
From Middle English anteme, from Old English antefn, antefen and Old French antiene, anteine, anteivne, from Latin antiphōna, from Ancient Greek ἀντίφωνα (antíphōna), from ἀντί (antí, "over against") + φωνή (phōnḗ, "voice, sound"). Doublet of antiphon.