To be plain, and because there seems to be controversy based on speculation rather than evidence, while it is possible that an idiom with an entirely ironic origin might become established, it is unlikely at best.
After that, ironic and nonironic uses both appeared in the same years, with the exception of no ironic uses and no nonironic uses. Ironic uses were the only usesfollowed by only nonironic usesand possibly Couched in ersatz dialect as this is, and supposing the author to be playing a long game, the phrase could be interpreted as an example of the character's misuse of a phrase that would ordinarily be ironic.
One can imagine during this period if the term was regularly used in this way, it could quickly gain a reputation for being a promise destined to be unfulfilled.
If I would speculate more, I would say it came from a "fat calf" being the calf that was ripe for the harvesting. Through highly visual lessons and guided practice, this course explores the quantitative reasoning behind probability and the cumulative nature of mathematics by tracing probability and statistics back to a foundation in the principles of counting.
I'm entertaining objections to the interpretation, either way.