Obviously not the answer, but I’ve got to show some love for Atlanta Braves catcher Bruce Benedict.
Atlanta fans always greeted him with a resounding BRUUUUUUUUCE! when he came to the plate. He was a pretty good defensive catcher & a fan favorite in spite of his light hitting.
I was actually at the game when he hit a grand slam off Fernando Valenzuela. You would have thought we had won the World Series from the way the crowd reacted.
Huh… I did a cursory google search and whaddya know was the second result? A thread from this year, on this very board.
(To be clear, I’m not disparaging you for not searching, just throwing it out there for more data points)
I’ve been looking at Game 7 of that '82 Series, and when Bruce Sutter got up to warm up in the 'pen, the Busch crowd would be saying “Bruce” or “Sut” or what have you, IIRC.
1332 B.C. Tutankhaman
“Tooooooooooot!!”
1754 B.C. Mesopotamia: “Enkidooooooooo!”
I was gonna say, when I started this exact same thread a little while ago, the best answer seeemed to be Bill “Moose” Skowron. His MLB career started in 1954 while Boog Powell’s started in 1961, so it’s reasonable to assume the “Moooose” chant may have been the first.
On David Letterman’s NBC show, whenever a joke got booed (usually because the audience thought it was in bad taste), Dave would say “they’re not booing, they’re chanting ‘Dave.’”
Philadelphia Flyers, early seventies, Andre “Mooooooooooose” DuPont.
My first recollection of the explanation dates to when Piniella was with the Royals, and Harry Jones said on a local Cleveland broadcast of an Indians’ game from Kansas City that "those are not boos, they’re saying “Looooooou!” A check of this page and Piniella’s establishes the time frame as somewhere between 1969 and '73 – and I’d guess '70 or '71, as I’m pretty sure Dave Martin was Harry’s partner and that the game was thus at the old Municipal Stadium in KC.
4000 BC, Sumeria. “Brewwwwwwwwwww!!!”
One of my favorite baseball players: Steve Buechele.
Or should I say: “Now batting… Steve Booooooooooo Chellllle!”
Stupid Rangers, I hated when Beuchele left and they brought up Dean Palmer. 
Deuce McAllister’s “Duuuueeece” sounds a lot like like a boo.
Just saying.
Moooooooooose! Damn I loved those Bullies. Too bad the NHL was bought out by the Ice Capades a few years later.
And then there’s Johnnie LeMaster, who really was being booed, and did something memorable about it.
OP, you were just thinking about this because of Jay Bruce, weren’t you?
Steve Smith, every time he touched the puck in Calgary after the 1986 playoffs. SHOOOOT!
Oddly enough even when he played for the Flames for a couple of seasons.
That’s who sparked the question, yes.
I wish I remembered the details of this story. Years ago I watched a ballgame on TV with one of those players with the “OO” sound in his name. Let’s call him Bruce. The guy made a bad play in the field, and the announcer said, “They’re not saying, ‘Bruce.’ They’re just booing. . . I’ve always wanted to say that.”
Royals fans are now cheering “Moose” for Mike Moustakas.
Jeff Francoeur, though, they’re genuinely booing.
I remember watching Goose Gossage stink up the joint when he was with the Cubs. Harry Caray tried to say that the crowd wasn’t booing, they were saying “Gooooooose!” Nice try, Harry.