Slots in after a hobby or pursuit to praise it: '[doing X], 其乐无穷'. Always positive, often used about hobbies, study, family life, or sports.
memory
Decode it: 其 'its' + 乐 'joy' + 无穷 'no end' - 'the joy of it has no end'. The 无穷 part also appears in 后患无穷 (lasting disaster) - same suffix, opposite vibe.
