1.
to assume, to take for granted, to decide firmly
To decide something firmly without proper evidence or consideration. Often implies an unfounded or premature assumption.
安全だと決め込んでいた。
I had assumed it was safe.
彼は来ないと決め込んでいた。
I had assumed he wouldn't come.
大丈夫だと決め込んで準備を怠った。
I neglected preparations, having assumed everything would be fine.
2.
to pretend, to feign, to play (a role)
To deliberately adopt a particular attitude or pretend to be in a certain state, often to avoid responsibility or confrontation. Commonly used with states like sleeping, ignorance, or innocence.
寝たふりを決め込む。
To pretend to be asleep.
知らないふりを決め込んだ。
I played dumb.
彼は何を聞かれても黙りを決め込んでいる。
No matter what he's asked, he insists on keeping silent.
USAGE:
A compound verb combining 決める (to decide) and 込む (into, thoroughly). The 込む suffix adds nuance of firmly settling into a state.
Sense 1 often implies the assumption was unwarranted or led to problems. Sense 2 is very common in the pattern Xを決め込む, where X is a pretended state.
COMMON PATTERNS:
- 寝たふりを決め込む: to pretend to be asleep
- 知らないふりを決め込む: to play dumb
- 黙りを決め込む: to clam up, to insist on silence
- 高をくくって決め込む: to take something lightly and assume