na-adjective, noun
Describes a person who is greedy for food or material things. Often used in a mildly scolding or teasing way. The word is somewhat colloquial and carries a negative, but not extremely harsh, tone.
あの子はいやしん坊で、いつもお菓子を欲しがる。
That kid is greedy and always wants snacks.
いやしん坊だから、食べ放題に行くと元を取ろうとしすぎる。
Because I'm a glutton, I try too hard to get my money's worth at all-you-can-eat places.
Derived from the adjective 卑しい (いやしい, 'greedy, lowly') plus ん坊 (a suffix meaning 'person' or 'child', often used in nicknames). The exact historical development is uncertain, but the word has been used in colloquial Japanese for centuries.