Genesis 18:3

(3) My lord.—Heb. ‘donai, a term of simple respect, just as the bowing towards the earth is exactly what an Arab sheik would do now to a passing traveller. Abraham’s conduct is marked by all that stately courtesy usual among Orientals. He calls himself their slave: regards it as a favour that they should partake of his hospitality; speaks slightingly of the repast prepared as a mere morsel of bread; and treats it as a providential act that they had come into his neighbourhood. It was only afterwards that he knew that he was entertaining angels unawares (Hebrews 13:2). While, moreover, he addresses the chief traveller first, as courtesy required, he immediately afterwards changes to the plural, lest he should seem wanting in hospitable welcome to his companions.