Commentary for Sanhedrin 49:20

Neither one of them [became] a nazirite - Two men were sitting and a man passed by. One said, “What’s-his-name who just passed is a nazirite!” and the other said “He’s not a nazirite! If he turns out to be a nazirite, I’m a nazirite!” His fellow said, “If he turns out not to be a nazirite, I’m a nazirite!” Both of them intended to take nazirite vows in the same way as one of them [intended to do so--which is to say, not at all]. Rabbi Tarfon said neither of them [became] a nazirite because his words were in jest and that is not a solemn vow; a person must make a solemn vow [to become a nazirite]. He [Rabbi Yehuda] is explaining Rabbi Tarfon’s reasoning; that is to say, he’s laying it out even though [the opinion] is not his. Even so, it is an asmachta [insincere and non-binding exchange] and they have not said enough for it to result in a legal acquisition.