There are many unreasonable things to feel resentment about at work.
This is the noun version of the kanji resent, so it means resentment.
Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You didn't learn that reading with this kanji, so here's a mnemonic to help you:
You feel some pretty strong resentment toward your roommate because you had to touch their icky door (いきどお) today and it was covered in what you hope was glue. Or frosting. UHG what an icky door. This is an outrage!!!