source of worry; cause of distress
Refers to a person, situation, or thing that is a persistent worry or trouble. Often used in everyday conversation.
His health is a source of worry.
Money is always a cause of distress.
Compound of 悩み (worry) + の (possessive particle) + 種 (seed), literally 'seed of worry'. The metaphor of a seed growing into a larger problem is common in Japanese.