If associate or deal with a particularly malicious, immoral, or unscrupulous person or people, you should be prepared that they will try to deceive, exploit, or harm you. Primarily heard in UK. The prime minister has been bending over backwards to curry the favour of the despotic leader, presumably to try and negotiate a trade deal that will give Britain access to their huge oil reserves. But as the old saying goes, he who sups with the devil should have a long spoon.