meaning of "slug" in a phrase

I've been rereading Heinlein's "Notebooks of Lazarus Long" recently and came across this phrase:

People who go broke in a big way never miss any meals. It is the poor jerk who is shy half a slug who must tighten his belt.

I understand what "shy (of)" means but I've checked dictionaries and can't figure out what "slug" refers to here. They propose meanings similar to this list:

1. mollusk

2. slow/lazy person

3. bullet or a cylindrical piece of metal

4. shot of an alcoholic drink

I don't really see any of them fitting here. From the way it's said it seems to mean something (not very) valuable since the "poor jerk" is not "broke in a big way" but still has to "tighten his belt" = go without a meal or two.

IIRC the original books date from 1950s-60s. The original book (Time enough for love) was written in the early 1970s.

Solution 1:

Given the context -- a galactic traveler dispensing bits of wisdom -- I always took slug to mean a generic unit of currency because the concept applies on any planet.

Even here on Earth, we have dollars, pounds, euros, yen, pesos, rubles, and so on, so if one were to give the same advice here, you'd probably pick a slang term that could be applied to any -- buck, quid, etc.

Perhaps, in this specific context, the term slug could be replaced by the equally generic coin or unit or credit, all of which I've seen used in various SF novels.

Solution 2:

Prior to the 1980s, vending machines were less sophisticated at detecting simple washers and stamped disks called:

slugs

A slug is a counterfeit coin that is used to make illegal purchases from a coin-operated device, such as a vending machine, payphone, parking meter, transit farebox, copy machine, coin laundry, gaming machine, or arcade game. -Wikipedia

So I think the reference is to a less than complete piece of worthless metal, or in other words, almost nothing. The "poor jerk" is just another guy who did not go broke: he already was.