Means done in a rushed, careless way, like you just chucked it together and hoped nobody would notice. If someone says your work is slapdash, they’re saying it’s messy, half-arsed, and lacking any attention to detail. Not exactly rare slang, but it’s a proper handy word for everyday moaning.
"This paint job’s proper slapdash, mate, you’ve left brush marks like zebra stripes. I only had ten minutes. Yeah, and it looks like you did it with a mop from Poundland"