Someone who's piggish is disagreeable and greedy. No one wants to be friends with the piggish kid at the party who shoves everyone else out of the way to grab all the candy from the piñata.
This adjective is perhaps a little unfair to pigs, who eat a lot but rarely more than they need. Before the 20th century, a piggish person was "unclean or coarse," another insult to pigs, known to roll in mud in order to stay cool and screen their skin from the sun. Today, the word is more commonly used to mean "greedy."