barbital + shanties = throbbed

Encrypting the plaintext ``barbital'' with the key ``shanties'' using the simple cipher a=(leave char unchanged) b=(forward one letter in the alphabet) b=(forward two letters)... yields the ciphertext ``throbbed'' which is the longest (and unique instance of length 8) in Linux's /usr/share/dict/words in which all three are English words. It also works swapping the plaintext and key.