In Elizabethan slang, it means 'wooden head' or 'block head'. It comes from 'clodpoll'.
The word 'clatpole' is used in Shakespeare's Troilus and Cressida, Act 2, scene 1, lines 110–120.
I shall cut out your tongue.
'Tis no matter, I shall speak as much as thou afterwards.
No more words, Thersites, peace!
I will hold my peace when Achilles' brach bids me, shall I?
There's for you, Patroclus.
I will see you hang'd like clatpoles (clotpole) ere I come any more to
your tents. I will keep where there is wit stirring, and leave the
faction of fools. *Exit*
A good riddance.