What is " lowball"

Do any of you have an idea of what constitutes lowball as a general rule. I have purchased things at 75% of asking price and have been ignored at 90%. All my sales have involved some haggling to find a middle ground and some started at what I thought was a lowball, so it annoys me when I am ignored. I guess it is better than getting involved with someone too busy to type NO THANKS and hit send. I'm just curious what everyone considers a rule of thumb in regards to making offers.