Probably variant (perhaps influenced by lizard and or gizzard) of Scots ezed, variant of zed.
OUR LIVING LANGUAGE:
The curious form izzard, meaning “the letter z,” sounds a lot like words such as lizard and gizzard. That's because izzard was probably modeled on them to form a humorous version of the name of the letter that appears as zed in Great Britain and ezed in Scotland. Izzard is practically limited to certain fixed regional expressions such as from A to izzard, “from beginning to end,” and not to know A from izzard, “not to know even the most basic things.”
z
rd
