Definition of 'Pig'
An investor who is often seen as greedy, having forgotten his or her original investment strategy to focus on securing unrealistic future gains. After experiencing a gain, these investors often have very high expectations about the future prospects of the investment and, therefore, do not sell their position to realize the gain.
Investopedia explains 'Pig'
Like a pig in the farmyard that overindulges in feed, this type of investor will hold onto an investment even after a substantial movement in the hope that the investment will provide even greater gains.
For example, suppose Joe invests in XYZ Corp. because the stock is undervalued. After the stock doubles its price in two months, Joe holds on to the whole investment, hoping that it will double again in the next two months, instead of selling a portion of the investment to realize a gain. Joe is, therefore, a piggish investor because he is greedy for huge gains and he allows his greed to supersede his original value investment strategy.