From mesitēs (mediator). To interpose, to act as a go-between, to mediate or guarantee. God Himself mediated by swearing an oath, since there was no one greater to swear by.
In Hebrews 6:17, God 'confirmed it with an oath' — He mediated/guaranteed His promise by swearing upon Himself. This is extraordinary: the God who cannot lie added an oath to His promise so that by two unchangeable things (His word and His oath), believers might have strong encouragement and absolute assurance. God accommodates human weakness by providing extra confirmation of what was already certain.