When people talk about words failing them, they normally have something vaguely semantic in mind, a feeling that words and meanings are out of joint, that they know what they want to say but not how to say it. This explanation is predicated on a notion that meaning exists prior to and independently of language, and whether or not that is the case is questionable. But whatever its cause, the...
In English, zo is not a very useful word. In Scrabble, ZO is the only eligible two-letter word with a Z in it: this makes it almost as useful as QI (neither, incidentally, is allowed in the United States). SQUALID in Scrabble may have nothing to do with DIRTY apart from an I and a D, but it has meaning nonetheless: it once meant for me 136 points (my best score ever, and much less than the 302 that the British champion Brett Smitheram – a student at Exeter University whose other hobby is bowls – once got for QUATORZES).