yvanspijk, 
'Duke' comes from Latin 'dux' (leader).
It's related to 'dūcere' (to lead; to pull), whence '-duce', e.g. 'to seduce' (i.e. to lead astray).
The 2nd part of German 'Herzog' (duke) is cognate to 'dux'. It's related to 'ziehen' (to pull), cognate of 'dūcere'.
Old English had cognates of both words.
Its counterpart of 'Herzog' was 'heretoga' (army leader). Middle English 'heretowe' would've become *hartow.
The Old English cognate of 'ziehen' was 'tēon'. This verb would've become *to tee.
