'Turnham' possibly recalls a homestead or watery meadow by a bend in the Thames.
Londonist takes '-ham' from Old English hamm, a watery, meadowy place, and says 'Turn-' supposedly reflects the twisting river; British History Online says the name possibly denoted a homestead by a bend in the river. It was recorded as Turneham in 1235 and Turnhamgrene in 1369; a Stephen of Turnham, recorded in 1199, probably took his name from it. In 1590 'Turnham Green common' was the name of waste land along the high road.