Thayer's Greek Lexicon

 1. to dip repeatedly, to immerse, to submerge (of vessels sunk)
2. to cleanse by dipping or submerging, to wash, to make clean
with water, to wash one's self, bathe
3. to overwhelm

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Not to be confused with G0911, bapto. The clearest example that shows
the meaning of baptizo is a text from the Greek poet and physician
Nicander, who lived about 200 B.C. It is a recipe for making pickles
and is helpful because it uses both words. Nicander says that in
order to make a pickle, the vegetable should first be 'dipped'
(bapto) into boiling water and then 'baptised' (baptizo) in the
vinegar solution. Both verbs concern the immersing of vegetables in a
solution. But the first is temporary. The second, the act of
baptising the vegetable, produces a permanent change.
When used in the New Testament, this word more often refers to our
union and identification with Christ than to our water baptism. e.g.
Mark 16:16. 'He that believes and is baptised shall be saved'.
Christ is saying that mere intellectual assent is not enough. There
must be a union with him, a real change, like the vegetable to the
pickle!
Bible Study Magazine, James Montgomery Boice, May 1989.