She'll be 91 years old in December. She lives alone in an efficiency apartment near Dupont Circle. A couple of days a week, she goes to the offices of Common Cause to do volunteer envelope-licking. The staff loves having her there. She loves being there. To be busy and productive is her lifeline.

But a D.C. cabbie--a greedy, gotta-look-out-for-Number-One cabbie -- may have put an end to the one thing that's keeping an old lady going.

On her "Common Cause days," the lady has been walking a block from her apartment house to the Fairfax Hotel. There's almost always a line of cabs waiting there, so the lady has almost always been able to hop in the first one and make the six-block, $1.45 trip with a minimum of difficulty. Coming home, she usually gets a ride from an officemate.

But cabbies don't sit on the Fairfax Hotel stand to take old ladies six blocks. They sit there waiting for executives who want to go to Dulles Airport. They sit there waiting for tourists who want to blow $40 for a quick spin around the monuments. They sit there waiting for anybody but an old lady who doesn't tip very much -- because she doesn't have very much to tip with.

Now, cabbies certainly have the right to complain if a low-tipping old lady gets in -- and complain they do.

But they don't have the right to blackball her. And that, sadly, is what one guy has done.

After "drawing" the Common Cause lady one day, he put out the word among the regulars on the Fairfax line:

She's nothing but a $1.45 fare.

Don't take her.

Lie to her.

Tell her that unless she's staying at the hotel, she can't use any of the cabs in the Fairfax line.

This happened about 10 days ago as you read this. The Common Cause lady now has no easy way to get to the office.

She could hail a cab on the street, but she finds that difficult and discouraging. She could take the bus, but at her age, she finds climbing the stairs a struggle. She could walk, but what about the days when it rains or snows?

The easy way out: don't go to Common Cause any more. The likely outcome: exactly that.

Common Cause will get along without the lady if it has to. But the opposite may not be true.

Her daughter says she's never seen her mother so down in the dumps. The staff at Common Cause is furious. The woman's friends are as upset as she is.

But somewhere around town, there's a cabbie who's all smiles. No more cheap trips for an old lady. More Dulles runs! More BWI runs! More $20 tippers trying to make the curtain at the Kennedy Center! Goody gum drops!

I hope somebody tips you in wooden nickels, pal.