adverb, na-adjective, noun
with all one's might; very hard; eagerly
Used to describe doing something with maximum effort, determination, or desperation. Can function as an adverb, adjectival noun, or noun.
彼は一生懸命勉強して、試験に合格した。
He studied very hard and passed the exam.
一生懸命走ったが、バスに間に合わなかった。
I ran as hard as I could, but I missed the bus.
彼女は一生懸命な人だから、きっと成功する。
She is a very hardworking person, so she will surely succeed.