A person that is shorter than the average male for his era and has an attitude because of his stunted growth. Any human male under 5'9" is classed as a 'short-arse'. Only when they have the related attitude problem (otherwise known as 'short-man-syndrome') can they class themselves as a short-arse-twat.
Most short-arse-twats normally try to 'big up' their actions, have loud unbearable ringtones and pretend to know it all in a bid to get themselves noticed.
Barry: "This guy came up to me the other day and said he had pulled 3 girls the night before and shagged them all throughout the night."
Paul: "Do you believe him?"
Barry: "Not really."
Paul: "Was he short?"
Barry: "Yes he was"
Paul: "don't worry about it, the 'short arse twat' would have been lying to impress you.