Hap is an old and now rare word meaning luck or chance — something that happens 'by hap' happens by chance, not by plan. You will almost never see 'hap' used by itself in modern English, but you already know it, hidden inside many common words: 'happen' (to occur by chance), 'happy' (originally, lucky), 'hapless' (unlucky), 'haphazard' (done by chance, not organized), and 'perhaps' (by chance).