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).
He studied hard, making the most of every spare moment.
It was such a busy day that I didn't have a moment's free time.
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).