It's a casual way to describe feeling well physically and mentally.
Origin:
The idiom likely originated in the early 20th century, when "A1" was used as a mark of quality. This designation was used on various products and services to indicate their high standard.
Examples:
* "I'm feeling A1 today, ready to tackle anything!"
* "After a good night's sleep, I woke up feeling A1."
* "I'm not feeling A1, I think I might be coming down with something."
Note: "A1" is often used in a similar way to "tip-top" or "in tip-top shape."