Baptism means to immerse. But, it does not always mean to be completely covered. Xenothon used the word to describe the wading of soldiers through a river. They were immersed, but only so far.
If I am not mistaken the word was used in the Septuagint in the cleansing ritual of houses in the old testament that were found to have some uncleaness. In that case it wasn't defined as immersion the way we immerse people.
(Lev 14:51) And he shall take the cedar wood, and the hyssop, and the scarlet, and the living bird, and dip them in the blood of the slain bird, and in the running water, and sprinkle the house seven times:
(Lev 14:52) And he shall cleanse the house with the blood of the bird, and with the running water, and with the living bird, and with the cedar wood, and with the hyssop, and with the scarlet:
(Lev 14:53) But he shall let go the living bird out of the city into the open fields, and make an atonement for the house: and it shall be clean.
(Lev 14:51) And he shall take the cedar wood, and the hyssop, and the scarlet, and the living bird, and dip them in the blood of the slain bird, and in the running water, and sprinkle the house seven times:
(Lev 14:52) And he shall cleanse the house with the blood of the bird, and with the running water, and with the living bird, and with the cedar wood, and with the hyssop, and with the scarlet:
(Lev 14:53) But he shall let go the living bird out of the city into the open fields, and make an atonement for the house: and it shall be clean.
(Lev 14:51) And he shall take the cedar wood, and the hyssop, and the scarlet, and the living bird, and dip them in the blood of the slain bird, and in the running water, and sprinkle the house seven times:
(Lev 14:52) And he shall cleanse the house with the blood of the bird, and with the running water, and with the living bird, and with the cedar wood, and with the hyssop, and with the scarlet:
(Lev 14:53) But he shall let go the living bird out of the city into the open fields, and make an atonement for the house: and it shall be clean.
Quick check on my handy Septuagint shows the word as βάψει or Bapsei. The AV translates that as "dip:" "and dip them in the blood of the slain bird. . . ."
The word for clean in v. 53 looks like καθαρός or katharos.
I can't make out "cleanse" in v. 52, but it looks something like aphagniei.