A foot cannon is a joke term for something which makes it easy to "shoot yourself in the foot". In other words a tool which is easy to use incorrectly.
Why is this term so familiar to people on HN but like OP I cannot find a single reference to it on Google...
I think people are very familiar with "foot gun" which is quite common, and are inferring that since a cannon is a really big gun, a foot cannon is a really big foot gun.
a more extreme foot-gun (this term should be more easily googleable)
A tool which is easy to accidentally misuse in ways that seriously harm yourself.
A way of catastrophically shooting oneself in the foot.