I can fend for myself. Well, sometimes. What I mean to say is that this is acceptable English usage.
I can fend off an attack. Well, sometimes. What I mean to say is that this, too, is acceptable, albeit different, English usage.
Archaically speaking, I can fend myself, although it would be more modern to defend myself. I can call on heaven to forfend something, if I insist on speaking archaically.
But while English allows me to fend, defend, and forfend, I cannot unfend. Indeed, fend is a sadly limited verb. Consider these strange omissions in its range. . .
I can prorate, but not profend. Upsell and upend, but not upfend. Counteract and countersign, but not counterfend.
I can disarm, disembark, and discontinue, but not disfend. Revisit, return, and refund, but not refend. Misstep, mislead, and misbehave, but not misfend.
I can (in usage, at least, if not in fact) outwait, outthink, and outperform, but not outfend. Become, bestir, and befriend, but not befend. Transact, transform, and transship, but not transfend. Cochair, cosign, and cohabitate, but not cofend.
For goodness sake, I can underestimate but not underfend, and overeat but not overfend, although I don’t for the life of me see why not.
I (well, Someone Else, p’raps) can preordain, but not even God can prefend. Thus is fend seen to be the opposite of preempt: I cannot empt, but I can do it early.
It’s all a bit of a puzzle, although I’m sure there are good historical reasons for all these limits on fend, if only I knew them. But as I search for ways to use my virtual tiles in Scrabble®, it seems cruel to be a hostage to history. After all, the computer just played “un” on my “gazed,” and that’s ridiculous.
“When I use a word,” Humpty Dumpty said, in rather a scornful tone, “it means just what I choose it to mean—neither more nor less.”
“The question is,” said Alice, “whether you can make words mean so many different things.”
“The question is,” said Humpty Dumpty, “which is to be master—that’s all.”
Lewis Carroll (Charles L. Dodgson), Through the Looking-Glass, chapter 6, p. 205 (1934)
Yeah! So back off, Scrabble® dictionary: I’m the master here. Cofenders is absolutely a word. What I mean to say is, “Fight me, fight my gang.”