In Yiddish FRAYER (literally the "free person") is the opposite of FRUM (i.e. religious, pious). The meaning "sucker" exists in contemporary Hebrew only. Hence the bilingual pun of the Yeshiva students who are apparently fluent in both these languages.
One student killed, three wounded at Arizona campus during fight (a)
from the article: Netanyahu is giving a kosher certificate to anti-Zionists