1. To humiliate, denigrate, or belittle someone. John is always doing people down as a way of making up for his own poor self esteem.

2. To cheat, swindle, or deceive someone. Primarily heard in UK. The huckster made a living from buying men a few too many rounds of ale, and then doing them down for whatever money they had on them. That jerk did me down, selling me a car that doesn't even work!