1. highly objectionable or offensive; odious: obnoxious behavior.
2. annoying or objectionable due to being a showoff or attracting undue attention to oneself: an obnoxious little brat.
3. Archaic. exposed or liable to harm, evil, or anything objectionable.
4. Obsolete. liable to punishment or censure; reprehensible.
I sit next to an obnoxious person that always farts. He's lactose intolerant, that's why.