Sweet Babboo is a pet name that Sally calls her crush Linus. It was inspired by Schulz's wife, Jean Schulz, who called her husband the same thing. Sally first calls Linus by the name in the strip from January 27, 1977.
This pet name seems to annoy Linus. Almost any time Sally mentions that Linus is her Sweet Babboo, Linus screams, "I'm not your Sweet Babboo!", whether or not he is visible in the strip or on screen. Once, when Linus was not around, Sally mentioned something to Charlie Brown about Linus being her Sweet Babboo, and Linus called her over the phone, and told her "I'm not your Sweet Babboo!". He does it again when Sally writes a letter and starts it "Dear Sweet Babboo,".
In the strip from February 12, 1991, Sally refers to herself as Linus' "Sweet Babbooette", to which Linus responded, "I've never heard of a 'Babbooette'!" In the strip from February 15, 1983 when Sally gets so annoyed about Linus ignoring her, she calls him, her "Sweet N' Sour Babboo".
The only other character who called Linus "My Sweet Babboo", was Eudora, to which Sally called out, "He's not your Sweet Babboo!". Eudora calling Linus her Sweet Babboo, did not seem to annoy Linus that much.
In the strip from December 20, 1998 (later adapted as a scene in the segment "Merry Christmas from Charlie Brown" in the 2002 animated special Charlie Brown's Christmas Tales), Charlie Brown makes a Christmas card for The Little Red-Haired Girl, writing on the card, "Merry Christmas, from your Sweet Babboo", to the annoyance of Linus, leading an embarrassed Charlie Brown to explain, "It's a family expression."