Charles H. Spurgeon on Psalms 108:9

Moab is my washpot. The office of washing the feet was in the East commonly performed by slaves, and the meanest of the family, as appears from what Abigail said to David when he took her to wife, "Behold, let thine handmaid be a servant to wash the feet of the servants of my lord", 1Sa 25:41; and from the fact of our Saviour washing his disciples' feet, to give them an example of humility, Joh 8:5. The word nipthr, used in this last passage, signifies in general a washing pot, and is put for the word podoniptron, the term which the Greeks, in strict propriety of speech, applied to a vessel for washing the feet. As this office was servile, so the vessels employed for this purpose were a mean part of household stuff. Gataker and Le Clerc illustrate this text from an anecdote related by Herodotus, concerning Amasis, king of Egypt, who expressed the meanness of his own origin by comparing himself to a pot for washing the feet in, (Herod. Lib. 2. c. 172). When, therefore, it is said, "Moab is my washing pot", the complete and servile subjection of Moab to David is strongly marked. This is expressed, not by comparing Moab to a slave who performs the lowest offices, as presenting to his master the basin for washing his feet, but by comparing him to the mean utensil itself. See 2Sa 8:2 1Ch 18:1-2, 12-13. —James Anderson's Note to Calvin on Isa 60:1-12.