1. What does it mean?
치맥 combines 치킨, usually Korean-style fried chicken in this context, and 맥주, beer. It names the pairing or a get-together centered on it.
Also written or said as chimaek, chicken and beer
2. The nuance
The word describes a food-and-drink combination, not every possible chicken meal. If somebody invites you for 치맥, it is still fine to ask for a nonalcoholic drink; the expression itself should not be read as pressure to drink.
3. Examples
“친구들이랑 치맥을 하면서 지난 여행 얘기를 했어.”
I had fried chicken and beer with friends while we talked about our last trip.
“오늘은 치맥 대신 치킨이랑 탄산수를 먹을래.”
Today I'll have chicken with sparkling water instead of beer.
Sources
KoreanClass101: Korean language lessons
References support the meaning and usage. Example sentences are written for NoCapWiki.