to dip something into a bowl or liquid: a brief, partial dipping, not a plunging under
This is a kitchen word before it is a Bible word. Greek writers used it for dipping cakes in honey, meat in brine, and beets in water, and doctors used it for steeping ingredients in a liquid. It is the ordinary verb "to dip" (βάπτω) with a short prefix meaning "in" fixed to the front, and scholars agree that the prefix adds very little: at most it spells out the "into" that the plain verb already implies. Greek of this period simply liked the longer, prefixed forms of common verbs.
That ordinariness matters, because one close relative of this word did become a religious term. The verb "to immerse" (βαπτίζω), which stands behind the English "baptize", carries the weight of ritual washing and Christian baptism. This word never made that move. It describes brief, partial contact: a hand or a piece of bread touched to what is in a dish, not a body put under water.
In the New Testament it turns up only twice, both times in the same moment. At his last meal Jesus says that the man dipping with him in the bowl is the one who will hand him over. Everyone at that table was reaching into the shared dishes, which is why the disciples keep asking whether he means them: the sentence does not point out a face. It says something worse. The traitor is a table companion, eating from the same bowl, and in that world sharing food was a pledge of trust and peace. The force of the scene rests on that gesture, and on the echo of Psalm 41:9, rather than on the choice of verb.