billowing up; rising thickly (of smoke, clouds, steam); (also used for working silently and steadily)
朝、工場の煙がもくもくと空に上がっていた。
In the morning, smoke from the factory was billowing up into the sky.
煮込み料理から湯気がもくもくと立っている。
Steam is rising thickly from the stew.
彼はもくもくと課題に取り組み、昼休みも忘れていた。
He worked steadily on the assignment, forgetting even his lunch break.