Green’s Dictionary of Slang

thick as... adj.

1. [late 16C+] stupid; used in a variety of combs., for the best-known of which see below.

2. [mid-18C+] very close, extremely intimate; used in a variety of combs., for the best-known of which see below.

3. [mid-19C+] (US) extremely plentiful; used in a variety of combs., for the best-known of which see below.

In phrases

...flies round a treacle pot (also ...flies in treacle, ...flies on a lump of sugar)

[mid-19C+] plentiful, numerous.

...pigshit

[1950s+] very stupid.

...poundies [Irish poundies, mashed potato mixed with onion and milk]

[late 19C–1900s] (Irish) very stupid .

...thieves (also ...two thieves)

[early 19C+] very intimate.

...three in a bed

[early 19C–1930s] very intimate.

...two inkle-weavers (also …inkle-weavers, ...two inkle-makers, as great as two inkle-weavers) [SE inkle, linen tape]

[mid-18C–early 19C] very close .

...two short planks (also ...a plank, ...eight short planks, …ten short planks, …two bricks, …two planks)

[1970s+] very stupid.