to circumcise: to cut away the foreskin as the physical sign that a male belongs to God's covenant people
In the New Testament this verb almost always describes the literal act: a Jewish boy circumcised on the eighth day (as with John and Jesus), Abraham circumcising Isaac, or Paul circumcising Timothy for the sake of his mission. It is the physical mark that set a male apart as belonging to God's covenant people, the practice Israel had kept since Abraham.
But in Acts and especially Paul's letters the same word sits at the center of the early church's sharpest fight. To "be circumcised" there is shorthand for something larger: taking on the entire law of Moses as the basis for standing with God. Paul says as much when he warns that a man who gets circumcised for that reason is bound to keep the whole law. The real question was whether Gentile believers had to become law-keeping Jews to be saved, and the word carries that whole controversy.
Colossians turns the word inward one more way: believers have been "circumcised with a circumcision made without hands," a spiritual cutting-away done by Christ rather than by a knife. The related noun (περιτομή) covers the rite, the resulting state, and even "the circumcised" as a group; the Hebrew verb behind the whole practice is (מוּל).