Abarim Publications' online Biblical Greek Dictionary
The verb νιπτω (nipto) means to rinse or wash off, mostly of a particular part of one's body (hands, feet, face), mostly directly after or right before some given activity (walking in from outside, getting ready to have a meal, and so on). This verb is actually a backformation from νιψω (nipso), the aorist form of the verb νιζω (nizo), in turn from the Proto-Indo-European root "nigw-", to wash or purge (which, curiously, also yielded the Germanic Nix, a malevolent water demon, which became the nickname of Nicolas, and somehow came to denote the element nickel; but all this in post-Biblical times).
Our verb νιπτω (nipto) emphasizes the rinsing off of loose dirt with water, rather than a immersive plunge (that's described by the verb βαπτιζω, baptizo) or a thorough deep-clean with soap (אזוב, 'ezob) in order to get wholly pure and clean (καθαρος, katharos).
Our verb is used 17 times in the New Testament, see full concordance, and from it derive:
- Together with the particle of negation α (a), meaning not or without: the adjective ανιπτος (aniptos), meaning unrinsed (Matthew 15:20, Mark 7:2 and 7:5 only).
- Together with the preposition απο (apo), meaning from: the verb απονιπτω (aponipto), meaning to rinse off of oneself. This verb is used in Matthew 27:24 only, where Pilate famously washed his hands off of whatever was being done to Jesus. This signifies two things. First, and most commonly recognized, Pilate purposed to declare himself innocent of what would happen next. But secondly, rather less commonly recognized, Pilate declared himself guilty of the blood of Christ, and anyone knows that a bit of rinsing doesn't remove blood from hands. By rinsing his hands in water, Pilate indicated that he wished to be innocent but immediately also confessed that he knew he would forever remain guilty.
- The noun νιπτηρ (nipter), meaning wash bowl (John 13:5 only).