The blues are about sex, of course, but the songs are also good-humoured and, at their best, suffused with a generosity of spirit. The band’s early radio hit, La Grange, was a hymn to a Texan house of ill repute.

The band broke through with the ineffably named Tush, a good-humoured but bruising guitar workout that turned a nonce word into a bawdy crowd-pleaser.

“That has a rather interesting backstory,” Gibbons says. “We were warming up for a show, way down in Alabama in a rodeo arena with a dirt floor. It was hot, and I started cranking out a warm-up guitar riff. The lighting director came running up and said, ‘Keep at it, whatever you’re doing is resonating!’

“We returned to the dressing room and grabbed a piece of paper. At the time down in Texas, the word ‘tush’ was kind of a slang word meaning fine or ‘the ultimate’ or ‘rico’ [Spanish for rich.] I said, ‘Well, let’s use this word – it means ‘the best!’ ”

It is suggested to Gibbons that “the best” probably is not what most people think the word means. He chuckles. “There are other connotations as well, which we’ll leave to one’s imagination.”

After the records in the 1970s that forever cemented the band’s reputation, ZZ Top pulled off a coup in the 1980s.

The two frontmen created a signature style, turning their beards, like their volume, up to 11, with whiskers down to belt buckles.

In the 1980s, the trio brightened their sound on singles like Legs and Sharp Dressed Man, and became comfortably cartoony MTV stars. Hill and Gibbons later paired the beards with a variety of outlandish headgear.

The decades since have flown by, and time, of course, has taken its toll on ZZ Top as it has on the music as a whole. Hill died of bursitis four years ago.

“It was a good run,” Gibbons says soberly. (Hill’s place is now taken by his longtime guitar technician, Elwood Francis, who has grown out his own beard.)

Along the way, Gibbons became a guitar hero. His conversation is peppered with reminiscences of this or that star. Ask Gibbons how it felt to play on stage with Eric Clapton, and he’ll note that he’d been talking to Clapton on the latter’s 80th birthday the week before. He’s crossed paths with many others, including Bob Dylan.

“One of my proudest possessions,” Gibbons says, “is a recording of Bob Dylan singing [ZZ Top song] My Head’s in Mississippi. He was playing in Mississippi. He turned to his band and said, ‘I hope you know it, because we’re going to do it!’ ”

His inspirations go back to when his mother took him to see Elvis Presley at the age of five. Rock and roll, he assures the interviewer, will continue as long as he has anything to say about it.

“It’s gotta save your soul, man,” Gibbons says, “I think that’s really true.”