IQ αμοιβάδας → dumb and dumber in one, dead slow, dead stupid, dumb, motorhead, complete and utter idiot, imbecile, dur-brain, numbnuts, numbskull, dimwit, dolt, blockhead, dumbbell, dumbo, slow on the uptake, dumber than a box of rocks, lamebrain, underhead, dunderhead

[dominotheory]

IQ αμοιβάδας → dumb and dumber in one, dead slow, dead stupid, dumb, motorhead, complete and utter idiot, imbecile, dur-brain, numbnuts, numbskull, dimwit, dolt, blockhead, dumbbell, dumbo, slow on the uptake, dumber than a box of rocks, lamebrain
   
IQ ραδικιού → dumb and dumber in one, dead slow, dead stupid, dumb, motorhead, complete and utter idiot, imbecile, dur-brain, numbnuts, numbskull, dimwit, dolt, blockhead, dumbbell, dumbo, slow on the uptake, dumber than a box of rocks, lamebrain
https://www.translatum.gr/forum/index.php?topic=240619.0

[eltheza]

as thick as two short planks (?)

https://www.google.co.uk/search?rlz=1C1GPEA_enGB318GB360&aq=0&oq=thick+as+two+short&sugexp=chrome,mod=13&ix=h9&sourceid=chrome&ie=UTF-8&q=thick+as+two+short+planks

[dominotheory]

That's a nice one eltheza - there's something roughly similar in Greek: τελείως κούτσουρο. In most cases, it's a reference to either illiterate or slow-witted people.

κούτσουρο
Το άτομο που δεν ξέρει τίποτα ή είναι χαμηλών πνευματικών δυνατοτήτων, ο σκράπας.
http://www.slang.gr/lemma/show/koutsouro_139

1α. μεγάλο κομμάτι από κορμό δέντρου ο οποίος έχει ξεραθεί και τεμαχιστεί: Kάθισε πάνω σ΄ ένα ~. || χοντρό καυσόξυλο: Tα κούτσουρα τριζοβολούσαν στο τζάκι. Έριξε ένα ~ στη φωτιά.
http://www.greek-language.gr/greekLang/modern_greek/tools/lexica/triantafyllides/search.html?lq=%CE%BA%CE%BF%CF%8D%CF%84%CF%83%CE%BF%CF%85%CF%81%CE%BF&dq=

[eltheza]

thicko, thickie, thicky (?) :-)

http://www.thefreedictionary.com/thicko