In theory, codenames are supposed to be random words that are in no way suggestive of the thing that they name. That ideal isn't always upheld, of course - the lure of a cute name can be too strong.
(For example it is suspected that FOGBANK, which is a codeword for a material used in the construction of nuclear physics packages, refers to a silica aerogel)
Reminds me of how Bletchley Park broke some of the Enigma ciphers. The operators were supposed to abide by certain rules when making the 3-letter "keys" -- but they inevitably picked letters next to each other on the keyboard, used the same letters repeatedly ("AAA"), used the same key "pairs", used words, names, etc.
(For example it is suspected that FOGBANK, which is a codeword for a material used in the construction of nuclear physics packages, refers to a silica aerogel)