What is the origin/meaning of "wheelbarrow full of frogs"
I've heard this phrase many times but am unsure as to exactly what it means.
A wheelbarrow is a smallish, open-topped, hand-propelled vehicle for the conveyance of bulky loads.
A frog is a small animal renowned for its jumping ability.
The frog is generally a very placid animal, but will jump to safety at the slightest provocation. If you place a bunch of frogs into a wheelbarrow in order to transfer the frogs from one place to another, then in order to arrive at your destination with any frogs left, you will have to be exceedingly careful not to bump, startle, or disturb the frogs in any way. And a wheelbarrow is not known for the smoothness of its ride, so your task is made all the more difficult by the choice of vehicle.
So the idea behind the idiom is that you have a task which is theoretically possible, but practical considerations render it either entirely unfeasible, or at the very least extremely difficult.
The idea of herding cats is very similar.
(I found one indication that the phrase is originally a "dutch metaphor", but no reliable source for that, so I'll let someone else dig into that side of things.)
Although you haven't given enough context to be sure, the idiom is usually mad as a box of frogs, which is to say, hopping mad.
Either the American (angry) mad or the British (insane) mad may be meant. I haven't heard a wheelbarrow before, but any container will presumably suffice, provided a lot of frogs can be fitted into it (albeit briefly).
The earliest example I found was in Usenet from a Mike Bartman's signature in a 7th March 1994 post to alt.tasteless.jokes:
This is very similar to an earlier saying from the 1980s: