in a moment, instantly, all of a sudden, neglect, disregard
忽 centers on something happening so quickly it is almost missed: in a moment, instantly, or all of a sudden. The neglect and disregard senses extend from the idea of treating something as fleeting or unworthy of sustained attention.
忽 combines 勿 (a shape associated with negation or cutting off) with 心 (heart/mind), suggesting a mental state where attention is cut off or fleeting. The exact historical development is uncertain.
The top 勿 looks like a quick, dismissive gesture, and the bottom 心 is the heart. Imagine your heart brushing something off in an instant, so fast that it is neglected.
For コツ, picture a sudden knock (kotsu) on the door that makes you jump in a moment: kotsu -> コツ, and the sound is so abrupt it is instantly noticed.
in an instant; in a moment; immediately; right away; at once
one hundred-thousandth
negligent; careless
to be negligent; to neglect; to slight
cannot be treated lightly; requiring careful attention; cannot be ignored
suddenly; abruptly; unexpectedly
sudden; unexpected
sudden; abrupt; unexpected
careless; thoughtless; rash; rude; at fault
careless person; thoughtless person
indiscreet; thoughtless