Definition of thick as thieves
- They were (as) thick as thieves for weeks, which made us wonder what they were doing.
: having or being of relatively great depth or extent from one surface to its opposite
: heavily built : thickset
: close-packed with units or individuals
: in a thick manner : thickly
: the most crowded or active part
: the part of greatest thickness
: one that steals especially stealthily or secretly
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