noun
tears shed in spite of oneself; involuntary tears
Used when someone cries without meaning to, often from being deeply moved or overwhelmed. The phrase implies a loss of composure.
感動のあまり、不覚の涙を流してしまった。
I was so moved that I shed tears in spite of myself.
Compound of 不覚 (fukaku, 'unintentional; loss of self-control') and 涙 (namida, 'tears'). The exact historical derivation is uncertain, but the phrase is conventionally associated with the meaning of crying involuntarily.