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1.
High Yield Investment Program.

A synonym for typically Internet-based ponzi schemes, with small minimums (typically less than US$500) and promised sky high returns without risk (i.e. more than 20% per year).

NOTE: See Triumph of the Optimists: 101 Years of Global Investment Returns
by Elroy Dimson, Paul Marsh and Mike Staunton.
Remember that new HYIP that I found that pays 25% a month? Well it paid for three months and then it collapsed.
by Alfredo Alberto September 20, 2004
 
2.
1. A general expression of excitement and happiness.

The word "hyip" is the bastard child of the words hip and yip. Usually repeated twice in quick succession, the pitch of your voice should match the level of your excitement (higher pitch = more excited). Users of the phrase are often seen attempting to leap into the air while sitting down.
- How do you like your Dell Axim x50v?
- Hyip! Hyip!
by E. Rock December 08, 2004