1. What does it mean?
소맥 takes 소 from 소주 and 맥 from 맥주. In drinking conversation it refers to mixing the two beverages, rather than a brand or a single fixed recipe.
Also written or said as somaek
2. The nuance
The word does not specify a ratio or alcohol strength. In other contexts 소맥 can mean wheat; the beverage sense depends on the setting.
3. Examples
“메뉴 설명에 소맥은 소주와 맥주를 섞은 술이라고 적혀 있어.”
The menu explains that somaek is a drink made by mixing soju and beer.
“친구는 소맥을 골랐고 나는 탄산수를 주문했어.”
My friend chose somaek, and I ordered sparkling water.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.