Oh dear, oilman T. Boone Pickens has apparently been Yanged.

That sounds like something that would happen out on the back 40 of a Texas ranch that no one in the clan ever discusses in polite company.

Actually, losing about $50 million is also something moneymen like Pickens don’t like to talk about either, even if he is as rich as can be.

Of course, in dumping his 10 million shares so precipitously, Pickens’s parting shot showed exactly what Yahoo’s big weakness is as the company heads into its annual meeting this Friday: an increasingly depressed stock.

Indeed, because of Pickens’s departure as an investor, Yahoo (YHOO) shares dropped just below the dangerous $20 a share level yesterday to $19.71, not far off the $19.18 price that prompted Microsoft (MSFT) to attack in February.

Yahoo shares are now just holding onto the edge at $20.05 this morning.

This is perhaps Yahoo’s biggest external problem. While the stock has shown resilience of really dipping to the mid-teens, if the shares drop that far, Pickens will look attractive compared with the kind of vulture investors who will show up to pick at Yahoo.

While Yahoo’s assets add up to $20 or more per share on a cash basis, staying above this threshold has largely been predicated on the fact that investors imagine some deal, any deal, between Yahoo and Microsoft in the future.

Such a deal might be the right move, but investors should probably accept the fact that it might never happen and that Yahoo and Microsoft will keep their digital efforts solo.

That’s the trap Pickens wandered right into like a particularly clueless bear.

And while Pickens took aim at Yahoo CEO Jerry Yang at an editorial board meeting with the San Francisco Chronicle yesterday, noting, “I think that Yahoo management was pathetic,” the fault is entirely Pickens’s own in greedily following along with activist investor and longtime crony Carl Icahn.

Pickens had bought a pile of shares in May, after Icahn announced his intention to wage a proxy fight against Yahoo and force it into a sale of some sort to Microsoft.

One problem: Yahoo and Microsoft execs did not go along with the grand idea of enriching either Pickens or Icahn, who aren’t exactly digitally inclined.

That’s no negative really, except to say that the Internet space is not quite the same as oil or, really, most other industries yet.

Despite the struggles of Yahoo and Microsoft against the more powerful Google (GOOG), the Internet remains a fast-growing industry with many options.

In other words, not quite ripe enough for takeover artists’, well, pickin’.

Today Pickens is wishing he’d known a bit more. Outraged over the company’s bungling of the Microsoft (MSFT) bid, the investor has sold his entire stake in Yahoo. At a loss. Said Pickens, “I think that Yahoo management was pathetic.”

Harsh words, and ones that will no doubt still be ringing in investors’ ears at Yahoo’s annual shareholder meeting for Friday.

Is it just me or did anyone else find the T. Boone Pickens (pictured here) quote about why he bought 10 million shares in Yahoo a little scary and a lot appalling?

“The only thing I know about Yahoo is what [CNBC Host] David [Faber] just told me, and that Carl Icahn is a big shareholder,” said the folksy chairman of BP Capital yesterday.

Yes, Icahn, the well-known Internet expert!

Actually, all the Luddite-leaning Icahn knows is how to squeeze money out of desperate and troubled companies (most of whom, to be fair, deserve the squeeze).

Which is why the entry into the game of all the corporate raider’s vulture-ish pals should be very, very disturbing to Yahoo CEO and Co-Founder Jerry Yang.

Because, whether or not Yahoo (YHOO) liked the unsolicited takeover incursion of Microsoft (MSFT), at least it is a technology company that valued and appreciated and understood the intricacies of Yahoo’s business.

Icahn, with all the delicacy of a meat grinder, would just as soon dismember Yahoo, if he got control and could not easily turn around and sell it off to Microsoft.

And, although there are a lot of lousy ways this situation could eventually turn out, that is perhaps the lousiest.

This morning Pickens, a legendary Texas oil and gas entrepreneur, said he’s bought up 10 million shares of Yahoo–a little less than 1% of the company. Why? Because Icahn invested in Yahoo first. “Carl Icahn is a smart guy,” Pickens said today in an interview with CBNC. “He does great things for shareholders and now I’m a shareholder. When I saw what he did, I jumped in behind him. … The only thing I know about Yahoo is what [CNBC Host] David [Faber] just told me, and that Carl Icahn is a big shareholder.”