moment's leisure; free minute
Refers to a very short period of free time, often used in expressions like 寸暇を惜しむ (to make the most of every spare moment).
It was such a busy day that I didn't have a moment's free time.
He studied hard, making the most of every spare moment.
I take a walk whenever I have a free minute.
He finds spare moments here and there to read books.
I used a spare moment to check my email.
余暇 refers to leisure time in general (like evenings or weekends), whereas 寸暇 emphasizes a tiny, momentary gap of free time.
Compound of 寸 (a very small unit of length, metaphorically 'a little') and 暇 (free time, leisure).