We found 8 dictionaries that define the word thick as two short planks:

General (3 matching dictionaries)

1. (as) thick as two short planks: Cambridge Advanced Learner's Dictionary
2. thick as two short planks: Wiktionary
3. thick as two short planks: TheFreeDictionary.com

Business (1 matching dictionary)

4. thick as two short planks: Legal dictionary

Medicine (1 matching dictionary)

5. thick as two short planks: Medical dictionary

Miscellaneous (1 matching dictionary)

6. thick as two short planks: Idioms

Slang (2 matching dictionaries)

7. thick as two short planks: English slang and colloquialisms used in the United Kingdom
8. thick as two short planks: Urban Dictionary

Definitions from Wiktionary (thick as two short planks) ▸ adjective:  (informal, simile) Very stupid. Similar: thick as a plank, thick, thick as a brick, thick as mince, thick as pigshit, dumb as a brick, thickwitted, thick-brained, dumb as a stump, dumb as a doorknob, more... Opposite: smart, intelligent, clever, astute, sharp Phrases: as thick as two short planks, more... ▸ Words similar to thick as two short planks ▸ Usage examples for thick as two short planks ▸ Idioms related to thick as two short planks ▸ Wikipedia articles (New!) ▸ Popular nouns described by thick as two short planks ▸ Words that often appear near thick as two short planks ▸ Rhymes of thick as two short planks ▸ Invented words related to thick as two short planks