1. What does it mean?
득근 combines 얻을 득, gain, with 근육's 근, muscle. Gym-goers use it for muscle gains or as a playful wish such as 득근하세요, roughly 'hope you make good gains.'
Also written or said as deukgeun
2. The nuance
It can simply signal encouragement between people who work out. It does not promise a measurable result from a particular session, supplement, or routine.
3. Examples
“운동 끝내고 친구한테 득근했냐고 장난쳤어.”
After the workout, I jokingly asked my friend whether they'd made some gains.
“단체 채팅방 인사는 오늘도 득근하세요였어.”
The group chat's greeting was 'Hope you make good gains today too.'
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.