So the mnemonic for 滞る kind of sucks and it’s more or less acknowledged in the mnemonic itself.
The reading is とどこおる and it means “To be overdue”… the current mnemonic reads:
“Just think of Tod (とど and some coot (こお) doing something wild even though they’re overdue for something else, or something.”
When there’s a glaring mnemonic right there in the reading!
Take the word for “to deliver” - とどく and the word for “to freeze” こおる and put them together! – "The DELIVERY was FROZEN and thus became OVERDUE!
glad i could help