Shortened version of 'hang on a minute' without actually confirming any time-scale at all.
Usually muttered when the person is busy on a computer / game console and is most often uttered without breaking eye contact with the screen or lifting of the head.
In many cases, the parent may give up waiting and carry out the task themselves which may result in serious repercussions so should be used fairly sparingly.
Mum "Tea's ready!" Son "Haminit"
Mum "Could you give me a hand unpacking the shopping?"
Mum "Ok, I'll get the chilled stuff into the fridge so that it doesn't exceed its sell by date"
Dad "Phone for you"
Dad "He'll phone you back in about an hour"