thick as thieves

From thick (“friendly, intimate”), first attested in 1827 as "thick as two thieves".

• "So the king sneaked into the wigwam and took to his bottle for comfort, and before long the duke tackled HIS bottle; and so in about a half an hour they was as thick as thieves again, and the tighter they got the lovinger they got, and went off a-snoring in each other's arms."
• "He and Victoria were as thick as thieves, and are about equal in wickedness."
• "President Bush may think he's as thick as thieves with his pal Vladimir Putin, but hopefully someone at the White House is reading the English edition of Pravda."