KanjiJLPT N1
Burdensome

burdensome, pack horse, horse load, send by horse, trivial, worthless

Mnemonic

A horse grown absurdly plump is utterly burdensome. Too fat to ride, too heavy to pull carts—it just stands there eating. Everyone agrees this plump horse is the most burdensome animal on the farm, a worthless drain on every last bale of hay.
Additional thoughts
Visualize a massively overweight horse that can't do any work. It eats everything, contributes nothing—pure dead weight. That uselessness is the core of burdensome. Plump horse equals pointless burden.
Quick recall
A plump horse is nothing but burdensome dead weight.

Details

The keyword for 駄 is burdensome. This kanji conveys the idea of something that is a load or burden, originally referring to a pack horse's load or the cargo carried on a horse's back. By extension, it came to signify something of poor quality, worthless, or useless—things that weigh one down without offering value. It is commonly associated with the sense of futility or wastefulness, as in something that serves no good purpose and is merely a burdensome weight.
On'yomi
だ、た