Refers to the impetuous, passionate, and sometimes reckless spirit of youth. Often used in literary or reflective contexts.
Actions driven by youthful ardor can sometimes lead to major failures.
His rash, impassioned remarks surprised everyone around him.
The exact derivation is uncertain; the kanji 客 (guest, outsider) and 気 (spirit, energy) combine to suggest an external, unrestrained force, fitting the sense of impetuous youthful passion.